Support scroll bar by extending the set of methods. (#272)

- compute(Horizontal|Vertical)ScrollExtent
- compute(Horizontal|Vertical)ScrollOffset
- compute(Horizontal|Vertical)ScrollRange

You can enable the scroll bar by adding the attribute to the RecyclerView.

E.g.
```xml
    <android.support.v7.widget.RecyclerView
        xmlns:android="http://schemas.android.com/apk/res/android"
        android:id="@+id/recyclerview"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:scrollbars="horizontal"
```

Note that the set of methods estimate the non-visible part of the flex container by the number of visible/non-visible items. So the returned values are not exact value of the container.
To calculate the exact value, we have to compute all the flex lines including even the non-visible part, which isn't practical since it may take considerable amount of time.

Fixes #249.

flexbox/src/main/java/com/google/android/flexbox/FlexboxLayoutManager.java

@@ -1128,11 +1128,6 @@ public class FlexboxLayoutManager extends RecyclerView.LayoutManager implements
         return getChildAt(0);
     }
 
-    private View getChildClosestToEnd() {
-        // TODO: Find from end when mFlexWrap == FlexWrap.WRAP_REVERSE
-        return getChildAt(getChildCount() - 1);
-    }
-
     /**
      * Fills the remaining space defined by the layoutState on
      * how many pixels should be filled (defined by {@link LayoutState#mAvailable}.
@@ -1948,6 +1943,127 @@ public class FlexboxLayoutManager extends RecyclerView.LayoutManager implements
         return referenceView;
     }
 
+    @Override
+    public int computeHorizontalScrollExtent(RecyclerView.State state) {
+        int scrollExtent = computeScrollExtent(state);
+        if (DEBUG) {
+            Log.d(TAG, "computeHorizontalScrollExtent: " + scrollExtent);
+        }
+        return scrollExtent;
+    }
+
+    @Override
+    public int computeVerticalScrollExtent(RecyclerView.State state) {
+        int scrollExtent = computeScrollExtent(state);
+        if (DEBUG) {
+            Log.d(TAG, "computeVerticalScrollExtent: " + scrollExtent);
+        }
+        return scrollExtent;
+    }
+
+    private int computeScrollExtent(RecyclerView.State state) {
+        if (getChildCount() == 0) {
+            return 0;
+        }
+        int allChildrenCount = state.getItemCount();
+        ensureOrientationHelper();
+        View firstReferenceView = findFirstReferenceChild(allChildrenCount);
+        View lastReferenceView = findLastReferenceChild(allChildrenCount);
+        if (state.getItemCount() == 0 || firstReferenceView == null || lastReferenceView == null) {
+            return 0;
+        }
+        // TODO: Need to consider the reverse pattern when flexWrap == wrap_reverse is implemented
+        int extend = mOrientationHelper.getDecoratedEnd(lastReferenceView) -
+                mOrientationHelper.getDecoratedStart(firstReferenceView);
+        return Math.min(mOrientationHelper.getTotalSpace(), extend);
+    }
+
+    @Override
+    public int computeHorizontalScrollOffset(RecyclerView.State state) {
+        int scrollOffset = computeScrollOffset(state);
+        if (DEBUG) {
+            Log.d(TAG, "computeHorizontalScrollOffset: " + scrollOffset);
+        }
+        return computeScrollOffset(state);
+    }
+
+    @Override
+    public int computeVerticalScrollOffset(RecyclerView.State state) {
+        int scrollOffset = computeScrollOffset(state);
+        if (DEBUG) {
+            Log.d(TAG, "computeVerticalScrollOffset: " + scrollOffset);
+        }
+        return scrollOffset;
+    }
+
+    private int computeScrollOffset(RecyclerView.State state) {
+        if (getChildCount() == 0) {
+            return 0;
+        }
+        int allChildrenCount = state.getItemCount();
+        View firstReferenceView = findFirstReferenceChild(allChildrenCount);
+        View lastReferenceView = findLastReferenceChild(allChildrenCount);
+        if (state.getItemCount() == 0 || firstReferenceView == null || lastReferenceView == null) {
+            return 0;
+        }
+        assert mFlexboxHelper.mIndexToFlexLine != null;
+        int minPosition = getPosition(firstReferenceView);
+        int maxPosition = getPosition(lastReferenceView);
+        int laidOutArea = Math.abs(mOrientationHelper.getDecoratedEnd(lastReferenceView) -
+                mOrientationHelper.getDecoratedStart(firstReferenceView));
+        int firstLinePosition = mFlexboxHelper.mIndexToFlexLine[minPosition];
+        if (firstLinePosition == 0 || firstLinePosition == NO_POSITION) {
+            return 0;
+        }
+        int lastLinePosition = mFlexboxHelper.mIndexToFlexLine[maxPosition];
+        int lineRange = lastLinePosition - firstLinePosition + 1;
+        float averageSizePerLine = (float) laidOutArea / lineRange;
+        // The number of lines before the first line is equal to the value of firstLinePosition
+        return Math.round(
+                firstLinePosition * averageSizePerLine + (mOrientationHelper.getStartAfterPadding()
+                        - mOrientationHelper.getDecoratedStart(firstReferenceView)));
+    }
+
+    @Override
+    public int computeHorizontalScrollRange(RecyclerView.State state) {
+        int scrollRange = computeScrollRange(state);
+        Log.d(TAG, "computeHorizontalScrollRange: " + scrollRange);
+        return scrollRange;
+    }
+
+    @Override
+    public int computeVerticalScrollRange(RecyclerView.State state) {
+        int scrollRange = computeScrollRange(state);
+        Log.d(TAG, "computeVerticalScrollRange: " + scrollRange);
+        return scrollRange;
+    }
+
+    /**
+     * @return the estimate total length of the flex container.
+     * Note that this method estimates the length by using the number of the visible items and the
+     * rest of the non-visible items. So the value returned by this method isn't the exact length
+     * of the container. So the scroll bar position may be slightly different from the actual
+     * expected position
+     */
+    private int computeScrollRange(RecyclerView.State state) {
+        if (getChildCount() == 0) {
+            return 0;
+        }
+        int allItemCount = state.getItemCount();
+        View firstReferenceView = findFirstReferenceChild(allItemCount);
+        View lastReferenceView = findLastReferenceChild(allItemCount);
+        if (state.getItemCount() == 0 || firstReferenceView == null || lastReferenceView == null) {
+            return 0;
+        }
+        assert mFlexboxHelper.mIndexToFlexLine != null;
+        int firstVisiblePosition = findFirstVisibleItemPosition();
+        int lastVisiblePosition = findLastVisibleItemPosition();
+        int laidOutArea = Math.abs(mOrientationHelper.getDecoratedEnd(lastReferenceView) -
+                mOrientationHelper.getDecoratedStart(firstReferenceView));
+        int laidOutRange = lastVisiblePosition - firstVisiblePosition + 1;
+        return (int) ((float) laidOutArea / laidOutRange * state.getItemCount());
+    }
+
     /**
      * Copied from {@link android.support.v7.widget.RecyclerView.LayoutManager#shouldMeasureChild
      * (View,