Run CSSOM smooth scroll animations on the compositor when possible

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 094edc3448faa393c93d9cd91049714cb4690fa4..d0c6b9c0a0ff38800fba8fb067859e19a5822e9f 100644
--- a/Source/platform/scroll/ScrollableArea.h
+++ b/Source/platform/scroll/ScrollableArea.h
@@ -65,7 +65,7 @@ public:
     virtual HostWindow* hostWindow() const { return 0; };
 
     bool scroll(ScrollDirection, ScrollGranularity, float delta = 1);
-    void scrollToOffsetWithoutAnimation(const FloatPoint&);
+    void scrollToOffsetWithoutAnimation(const FloatPoint&, bool cancelProgrammaticAnimations = true);
     void scrollToOffsetWithoutAnimation(ScrollbarOrientation, float offset);
 
     void programmaticallyScrollSmoothlyToOffset(const FloatPoint&);
@@ -206,9 +206,12 @@ public:
     // animations.
     bool scheduleAnimation();
     void serviceScrollAnimations(double monotonicTime);
+    void updateCompositorScrollAnimations();
     virtual void registerForAnimation() { }
     virtual void deregisterForAnimation() { }
 
+    void notifyCompositorAnimationFinished(int groupId);
+
     virtual bool usesCompositedScrolling() const { return false; }
 
     // Returns true if the GraphicsLayer tree needs to be rebuilt.