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

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

Index: third_party/WebKit/Source/platform/scroll/ScrollAnimatorCompositorCoordinator.cpp
diff --git a/third_party/WebKit/Source/platform/scroll/ScrollAnimatorCompositorCoordinator.cpp b/third_party/WebKit/Source/platform/scroll/ScrollAnimatorCompositorCoordinator.cpp
index 17aecd654842c4a129b25e54776c9fd0c7a843db..dfddca9c249e0c5f8e1cfd0a575151f313fb0973 100644
--- a/third_party/WebKit/Source/platform/scroll/ScrollAnimatorCompositorCoordinator.cpp
+++ b/third_party/WebKit/Source/platform/scroll/ScrollAnimatorCompositorCoordinator.cpp
@@ -150,9 +150,10 @@ void ScrollAnimatorCompositorCoordinator::compositorAnimationFinished(
     }
 }
 
-void ScrollAnimatorCompositorCoordinator::reattachCompositorPlayerIfNeeded(
+bool ScrollAnimatorCompositorCoordinator::reattachCompositorPlayerIfNeeded(
     WebCompositorAnimationTimeline* timeline)
 {
+    bool reattached = false;
     int compositorAnimationAttachedToLayerId = 0;
     if (scrollableArea()->layerForScrolling())
         compositorAnimationAttachedToLayerId = scrollableArea()->layerForScrolling()->platformLayer()->id();
@@ -171,10 +172,13 @@ void ScrollAnimatorCompositorCoordinator::reattachCompositorPlayerIfNeeded(
                 timeline->playerAttached(*this);
                 m_compositorPlayer->attachLayer(
                     scrollableArea()->layerForScrolling()->platformLayer());
+                reattached = true;
             }
             m_compositorAnimationAttachedToLayerId = compositorAnimationAttachedToLayerId;
         }
     }
+
+    return reattached;
 }
 
 void ScrollAnimatorCompositorCoordinator::notifyAnimationStarted(