Add isAdjustable to SemanticFlags (#2776)

sky/services/semantics/semantics.mojom

@@ -24,6 +24,7 @@ struct SemanticFlags {
   bool canBeScrolledVertically = false;
   bool hasCheckedState = false; // whether isChecked is relevant
   bool isChecked = false;
+  bool isAdjustable = false;
 };
 
 struct SemanticStrings {
@@ -69,4 +70,6 @@ interface SemanticsServer {
   ScrollRight(uint32 nodeID);
   ScrollUp(uint32 nodeID);
   ScrollDown(uint32 nodeID);
+  AdjustIncrease(uint32 nodeID);
+  AdjustDecrease(uint32 nodeID);
 };

sky/shell/platform/android/io/flutter/view/AccessibilityBridge.java

@@ -297,6 +297,7 @@ class AccessibilityBridge extends AccessibilityNodeProvider implements Semantics
                 canBeScrolledVertically = node.flags.canBeScrolledVertically;
                 hasCheckedState = node.flags.hasCheckedState;
                 isChecked = node.flags.isChecked;
+                isAdjustable = node.flags.isAdjustable;
             }
             if (node.strings != null) {
                 label = node.strings.label;
@@ -351,6 +352,7 @@ class AccessibilityBridge extends AccessibilityNodeProvider implements Semantics
         boolean canBeScrolledVertically;
         boolean hasCheckedState;
         boolean isChecked;
+        boolean isAdjustable;
         String label;
         List<PersistentAccessibilityNode> children;
 

sky/shell/platform/ios/framework/Source/accessibility_bridge.mm

@@ -127,10 +127,12 @@ struct Geometry {
 }
 
 - (UIAccessibilityTraits)accessibilityTraits {
-  // TODO(tvolkert): We need more semantic info in the mojom definition
-  // in order to distinguish buttons, links, sliders, etc.
-  return _flags->canBeTapped ? UIAccessibilityTraitButton
-                             : UIAccessibilityTraitNone;
+  UIAccessibilityTraits traits = UIAccessibilityTraitNone;
+  if (_flags->canBeTapped)
+    traits |= UIAccessibilityTraitButton;
+  if (_flags->isAdjustable)
+    traits |= UIAccessibilityTraitAdjustable;
+  return traits;
 }
 
 - (CGRect)accessibilityFrame {