Correctly handle cancelling compositor animations initiated from main thread.

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

 /*
  * 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 78 matching lines @@
 
 ScrollResultOneDimensional ScrollAnimator::userScroll(
     ScrollbarOrientation orientation, ScrollGranularity granularity, float step, float delta)
 {
     if (!m_scrollableArea->scrollAnimatorEnabled())
         return ScrollAnimatorBase::userScroll(orientation, granularity, step, delta);
 
     TRACE_EVENT0("blink", "ScrollAnimator::scroll");
 
     if (granularity == ScrollByPrecisePixel) {
-        if (hasRunningAnimation()) {
-            abortAnimation();
-            resetAnimationState();
-        }
+        // Cancel scroll animation because asked to instant scroll.
+        if (hasRunningAnimation())
+            cancelAnimation();
         return ScrollAnimatorBase::userScroll(orientation, granularity, step, delta);
     }
 
     float usedPixelDelta = computeDeltaToConsume(orientation, step * delta);
     FloatPoint pixelDelta = (orientation == VerticalScrollbar
         ? FloatPoint(0, usedPixelDelta) : FloatPoint(usedPixelDelta, 0));
 
     FloatPoint targetPos = desiredTargetPosition();
     targetPos.moveBy(pixelDelta);
 
-    if (m_animationCurve) {
+    if (m_animationCurve && m_runState != RunState::WaitingToCancelOnCompositor) {
         if ((targetPos - m_targetOffset).isZero()) {
             // Report unused delta only if there is no animation running. See
             // comment below regarding scroll latching.
             return ScrollResultOneDimensional(/* didScroll */ true, /* unusedScrollDelta */ 0);
         }
 
         m_targetOffset = targetPos;
         ASSERT(m_runState == RunState::RunningOnMainThread
             || m_runState == RunState::RunningOnCompositor
             || m_runState == RunState::RunningOnCompositorButNeedsUpdate);
@@ skipping 175 matching lines @@
     }
     return true;
 }
 
 DEFINE_TRACE(ScrollAnimator)
 {
     ScrollAnimatorBase::trace(visitor);
 }
 
 } // namespace blink