Add baseFont method to FontCollection

This patch adds a method to retrieve the base font from a
FontCollection, which is useful when querying global font metrics.

Part of the fix for bug 15467288 "Inconsistent line heights on
Minikin builds"

Change-Id: I268ae5128d0852a020d746bc22af81fc1a623228

include/minikin/FontCollection.h

@@ -57,6 +57,9 @@ public:
     void itemize(const uint16_t *string, size_t string_length, FontStyle style,
             std::vector<Run>* result) const;
 
+    // Get the base font for the given style, useful for font-wide metrics.
+    MinikinFont* baseFont(FontStyle style);
+
     uint32_t getId() const;
 private:
     static const int kLogCharsPerPage = 8;

libs/minikin/FontCollection.cpp

@@ -185,6 +185,14 @@ void FontCollection::itemize(const uint16_t *string, size_t string_size, FontSty
     }
 }
 
+MinikinFont* FontCollection::baseFont(FontStyle style) {
+    if (mInstances.empty()) {
+        return NULL;
+    }
+    return mInstances[0].mFamily->getClosestMatch(style);
+}
+
+
 uint32_t FontCollection::getId() const {
     return mId;
 }