Fix retargeting of composited smooth scrolls from the main thread.

third_party/WebKit/Source/platform/scroll/ScrollAnimator.h

 /*
  * Copyright (c) 2011, Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 47 matching lines @@
     void cancelAnimation() override;
     void resetAnimationState() override;
     void updateCompositorAnimations() override;
     void notifyCompositorAnimationFinished(int groupId) override;
     void layerForCompositedScrollingDidChange(WebCompositorAnimationTimeline*) override;
 
     DECLARE_VIRTUAL_TRACE();
 
 protected:
     OwnPtr<WebScrollOffsetAnimationCurve> m_animationCurve;
-    double m_lastTickTime;
     double m_startTime;
     WTF::TimeFunction m_timeFunction;
 
 private:
     FloatPoint desiredTargetPosition() const;
 
+    // Returns true if the animation was scheduled successfully. If animation
+    // could not be scheduled (e.g. because the frame is detached), scrolls
+    // immediately to the target and returns false.
+    bool registerAndScheduleAnimation();
+
     FloatPoint m_targetOffset;
 };
 
 } // namespace blink
 
 #endif // ScrollAnimator_h