Run CSSOM smooth scroll animations on the compositor when possible

Source/platform/scroll/ProgrammaticScrollAnimator.h

Index: Source/platform/scroll/ProgrammaticScrollAnimator.h
diff --git a/Source/platform/scroll/ProgrammaticScrollAnimator.h b/Source/platform/scroll/ProgrammaticScrollAnimator.h
index a37d9b374e94d6de1965a405b53f6586abea54c7..538077d0394922f78c6f2726bb27e2dacafaf276 100644
--- a/Source/platform/scroll/ProgrammaticScrollAnimator.h
+++ b/Source/platform/scroll/ProgrammaticScrollAnimator.h
@@ -29,17 +29,42 @@ public:
     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