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 b710d7d48efd7b1847df169a1666697405f0c095..95015f3c46415c89be4f45408e4a03ff41fa8f5d 100644
--- a/Source/platform/scroll/ScrollableArea.h
+++ b/Source/platform/scroll/ScrollableArea.h
@@ -40,6 +40,7 @@ class HostWindow;
 class PlatformWheelEvent;
 class ProgrammaticScrollAnimator;
 class ScrollAnimator;
+class ScrollbarStateTransitionAnimator;
 
 enum ScrollBehavior {
     ScrollBehaviorAuto,
@@ -107,7 +108,7 @@ public:
 
     virtual void contentsResized();
 
-    bool hasOverlayScrollbars() const;
+    virtual bool hasOverlayScrollbars() const;
     void setScrollbarOverlayStyle(ScrollbarOverlayStyle);
     ScrollbarOverlayStyle scrollbarOverlayStyle() const { return static_cast<ScrollbarOverlayStyle>(m_scrollbarOverlayStyle); }
 
@@ -123,6 +124,17 @@ public:
         return m_animators ? m_animators->programmaticScrollAnimator.get() : 0;
     }
 
+    // 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_animators ? m_animators->scrollbarStateTransitionAnimator.get() : 0;
+    }
+
+    bool isDuringStateTransitionAnimation() const;
+    void stateTransitionInProgress();
+
     const IntPoint& scrollOrigin() const { return m_scrollOrigin; }
     bool scrollOriginChanged() const { return m_scrollOriginChanged; }
 
@@ -192,6 +204,7 @@ public:
     // animations.
     bool scheduleAnimation();
     void serviceScrollAnimations(double monotonicTime);
+    void serviceScrollbarAnimations();
 
     virtual bool usesCompositedScrolling() const { return false; }
 
@@ -270,6 +283,7 @@ private:
     struct ScrollableAreaAnimators {
         OwnPtr<ScrollAnimator> scrollAnimator;
         OwnPtr<ProgrammaticScrollAnimator> programmaticScrollAnimator;
+        OwnPtr<ScrollbarStateTransitionAnimator> scrollbarStateTransitionAnimator;
     };
 
     mutable OwnPtr<ScrollableAreaAnimators> m_animators;