Blink Support for Overlay Scrollbar Animation

Source/platform/scroll/ScrollableArea.h

Index: Source/platform/scroll/ScrollableArea.h
diff --git a/Source/platform/scroll/ScrollableArea.h b/Source/platform/scroll/ScrollableArea.h
index 358b77b93f0cb7417d4b91607522580a5dc84f2f..050a6838dc8cb22cccd78ca6185f4f9f0ee06855 100644
--- a/Source/platform/scroll/ScrollableArea.h
+++ b/Source/platform/scroll/ScrollableArea.h
@@ -40,6 +40,7 @@ class GraphicsLayer;
 class PlatformGestureEvent;
 class PlatformWheelEvent;
 class ScrollAnimator;
+class ScrollbarStateTransitionAnimator;
 
 enum ScrollBehavior {
     ScrollBehaviorAuto,
@@ -101,7 +102,7 @@ public:
 
     virtual void contentsResized();
 
-    bool hasOverlayScrollbars() const;
+    virtual bool hasOverlayScrollbars() const;
     void setScrollbarOverlayStyle(ScrollbarOverlayStyle);
     ScrollbarOverlayStyle scrollbarOverlayStyle() const { return static_cast<ScrollbarOverlayStyle>(m_scrollbarOverlayStyle); }
 
@@ -111,6 +112,17 @@ public:
     // This getter will return null if the ScrollAnimator hasn't been created yet.
     ScrollAnimator* existingScrollAnimator() const { return m_scrollAnimator.get(); }
 
+    // This getter will create a ScrollbarStateTransitionAnimator if needed and doesn't already exist.
+    ScrollbarStateTransitionAnimator* scrollbarStateTransitionAnimator() const;
+    // This getter will return null if the ScrollbarStateTransitionAnimator hasn't been created yet.
+    ScrollbarStateTransitionAnimator* existingScrollbarStateTransitionAnimator() const
+    {
+        return m_scrollbarStateTransitionAnimator.get();
+    }
+
+    bool isDuringStateTransitionAnimation() const;
+    void stateTransitionInProgress();
+
     const IntPoint& scrollOrigin() const { return m_scrollOrigin; }
     bool scrollOriginChanged() const { return m_scrollOriginChanged; }
 
@@ -179,7 +191,7 @@ public:
     // Let subclasses provide a way of asking for and servicing scroll
     // animations.
     virtual bool scheduleAnimation() { return false; }
-    void serviceScrollAnimations();
+    void serviceScrollbarAnimations();
 
     virtual bool usesCompositedScrolling() const { return false; }
 
@@ -257,6 +269,7 @@ private:
     virtual float pixelStep(ScrollbarOrientation) const;
 
     mutable OwnPtr<ScrollAnimator> m_scrollAnimator;
+    mutable OwnPtr<ScrollbarStateTransitionAnimator> m_scrollbarStateTransitionAnimator;
     unsigned m_constrainsScrollingToContentEdge : 1;
 
     unsigned m_inLiveResize : 1;