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

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 241 matching lines @@
             int animationGroupId = animation->group();
 
             sentToCompositor = addAnimation(animation.release());
             if (sentToCompositor) {
                 m_runState = RunState::RunningOnCompositor;
                 m_compositorAnimationId = animationId;
                 m_compositorAnimationGroupId = animationGroupId;
             }
         }
 
+        bool runningOnMainThread = false;
         if (!sentToCompositor) {
-            if (registerAndScheduleAnimation())
+            runningOnMainThread = registerAndScheduleAnimation();
+            if (runningOnMainThread)
                 m_runState = RunState::RunningOnMainThread;
         }
+
+        // Main thread should deal with the scroll animations it started.
+        if (sentToCompositor || runningOnMainThread)
+            addMainThreadScrollingReason();
     }
 }
 
 void ScrollAnimator::notifyCompositorAnimationAborted(int groupId)
 {
     // An animation aborted by the compositor is treated as a finished
     // animation.
     ScrollAnimatorCompositorCoordinator::compositorAnimationFinished(groupId);
 }
 
@@ skipping 23 matching lines @@
     }
     return true;
 }
 
 DEFINE_TRACE(ScrollAnimator)
 {
     ScrollAnimatorBase::trace(visitor);
 }
 
 } // namespace blink