Run CSSOM smooth scroll animations on the compositor when possible

Source/platform/scroll/ProgrammaticScrollAnimator.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef ProgrammaticScrollAnimator_h
 #define ProgrammaticScrollAnimator_h
 
 #include "platform/geometry/FloatPoint.h"
 #include "wtf/FastAllocBase.h"
 #include "wtf/Noncopyable.h"
@@ skipping 11 matching lines @@
     WTF_MAKE_NONCOPYABLE(ProgrammaticScrollAnimator);
     WTF_MAKE_FAST_ALLOCATED;
 public:
     static PassOwnPtr<ProgrammaticScrollAnimator> create(ScrollableArea*);
 
     ~ProgrammaticScrollAnimator();
 
     void animateToOffset(FloatPoint);
     void cancelAnimation();
     void tickAnimation(double monotonicTime);
-    bool hasRunningAnimation() const;
+    bool hasAnimationThatRequiresService() const;
+    void updateCompositorAnimations();
+    void notifyCompositorAnimationFinished(int groupId);
 
 private:
     explicit ProgrammaticScrollAnimator(ScrollableArea*);
 
+    enum class RunState {
+        // No animation.
+        Idle,
+
+        // Waiting to send an animation to the compositor. There might also
+        // already be another animation running on the compositor that will need
+        // to be canceled first.
+        WaitingToSendToCompositor,
+
+        // Running an animation on the compositor.
+        RunningOnCompositor,
+
+        // Running an animation on the main thread.
+        RunningOnMainThread,
+
+        // Waiting to cancel the animation currently running on the compositor.
+        // There is no pending animation to replace the canceled animation.
+        WaitingToCancelOnCompositor
+    };
+
     void resetAnimationState();
 
     ScrollableArea* m_scrollableArea;
     OwnPtr<WebScrollOffsetAnimationCurve> m_animationCurve;
     FloatPoint m_targetOffset;
     double m_startTime;
+    RunState m_runState;
+    int m_compositorAnimationId;
+    int m_compositorAnimationGroupId;
 };
 
 } // namespace blink
 
 #endif // ProgrammaticScrollAnimator_h