Feed ScrollableArea::showOverlayScrollbars into ScrollbarAnimationController.

cc/input/scrollbar_animation_controller.h

Index: cc/input/scrollbar_animation_controller.h
diff --git a/cc/input/scrollbar_animation_controller.h b/cc/input/scrollbar_animation_controller.h
index e06f594423b3c6ad32cbae65960b952c33e2a93a..d481eae15fbc6467ab1408c5b1252359e024c320 100644
--- a/cc/input/scrollbar_animation_controller.h
+++ b/cc/input/scrollbar_animation_controller.h
@@ -85,6 +85,10 @@ class CC_EXPORT ScrollbarAnimationController {
   void DidMouseLeave();
   void DidMouseMoveNear(ScrollbarOrientation, float);
 
+  // Called when Blink wants to show the scrollbars (via
+  // ScrollableArea::showOverlayScrollbars).
+  void DidRequestShowFromMainThread();
+
   bool MouseIsOverScrollbar(ScrollbarOrientation orientation) const;
   bool MouseIsNearScrollbar(ScrollbarOrientation orientation) const;
   bool MouseIsNearAnyScrollbar() const;