Feed ScrollableArea::showOverlayScrollbars into ScrollbarAnimationController.

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

Index: third_party/WebKit/Source/platform/scroll/ScrollableArea.h
diff --git a/third_party/WebKit/Source/platform/scroll/ScrollableArea.h b/third_party/WebKit/Source/platform/scroll/ScrollableArea.h
index a284ce6b086391f8964305a3b635ef7cc64682e9..1a8f05794889170be1fc3ab442cb5e7f260202ff 100644
--- a/third_party/WebKit/Source/platform/scroll/ScrollableArea.h
+++ b/third_party/WebKit/Source/platform/scroll/ScrollableArea.h
@@ -304,6 +304,8 @@ class PLATFORM_EXPORT ScrollableArea : public GarbageCollectedMixin,
   bool hasLayerForScrollCorner() const;
 
   void layerForScrollingDidChange(CompositorAnimationTimeline*);
+  bool needsShowScrollbarLayers() const { return m_needsShowScrollbarLayers; }
+  void didShowScrollbarLayers() { m_needsShowScrollbarLayers = false; }
 
   void cancelScrollAnimation();
   virtual void cancelProgrammaticScrollAnimation();
@@ -446,6 +448,10 @@ class PLATFORM_EXPORT ScrollableArea : public GarbageCollectedMixin,
   unsigned m_scrollbarCaptured : 1;
   unsigned m_mouseOverScrollbar : 1;
 
+  // Indicates that the next compositing update needs to call
+  // WebLayer::showScrollbars on our scroll layer. Ignored if not composited.
+  unsigned m_needsShowScrollbarLayers : 1;
+
   // There are 6 possible combinations of writing mode and direction. Scroll
   // origin will be non-zero in the x or y axis if there is any reversed
   // direction or writing-mode. The combinations are: