Fix smooth scroll jump when switching scroll handling between MT and CC

third_party/WebKit/Source/platform/scroll/ScrollableArea.cpp

 /*
  * Copyright (c) 2010, Google Inc. All rights reserved.
  * Copyright (C) 2008, 2011 Apple 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.
@@ skipping 499 matching lines @@
     if (ScrollAnimatorBase* scrollAnimator = existingScrollAnimator())
         scrollAnimator->notifyCompositorAnimationAborted(groupId);
 }
 
 void ScrollableArea::cancelScrollAnimation()
 {
     if (ScrollAnimatorBase* scrollAnimator = existingScrollAnimator())
         scrollAnimator->cancelAnimation();
 }
 
+void ScrollableArea::clearMainThreadScrollingReasons(bool clear)
+{
+    if (ScrollAnimatorBase* scrollAnimator = existingScrollAnimator())
+        scrollAnimator->clearMainThreadScrollingReasons(clear);
+}
+
 void ScrollableArea::cancelProgrammaticScrollAnimation()
 {
     if (ProgrammaticScrollAnimator* programmaticScrollAnimator = existingProgrammaticScrollAnimator())
         programmaticScrollAnimator->cancelAnimation();
 }
 
 bool ScrollableArea::shouldScrollOnMainThread() const
 {
     if (GraphicsLayer* layer = layerForScrolling()) {
         return layer->platformLayer()->shouldScrollOnMainThread();
@@ skipping 73 matching lines @@
         std::max(0, size.height() - horizontalScrollbarHeight));
 }
 
 DEFINE_TRACE(ScrollableArea)
 {
     visitor->trace(m_scrollAnimator);
     visitor->trace(m_programmaticScrollAnimator);
 }
 
 } // namespace blink