Feed ScrollableArea::showOverlayScrollbars into ScrollbarAnimationController.

cc/layers/layer_impl.h

Index: cc/layers/layer_impl.h
diff --git a/cc/layers/layer_impl.h b/cc/layers/layer_impl.h
index c977c56e2ff04c76895a80f830e1d22fa82af492..15dcdbbac6762571095de95b72555d893b551c12 100644
--- a/cc/layers/layer_impl.h
+++ b/cc/layers/layer_impl.h
@@ -439,6 +439,9 @@ class CC_EXPORT LayerImpl {
 
   ElementListType GetElementTypeForAnimation() const;
 
+  void set_needs_show_scrollbars(bool yes) { needs_show_scrollbars_ = yes; }
+  bool needs_show_scrollbars() { return needs_show_scrollbars_; }
+
  protected:
   LayerImpl(LayerTreeImpl* layer_impl,
             int id,
@@ -556,6 +559,11 @@ class CC_EXPORT LayerImpl {
   bool needs_push_properties_ : 1;
   bool scrollbars_hidden_ : 1;
 
+  // The needs_show_scrollbars_ bit tracks a pending request from Blink to show
+  // the overlay scrollbars. It's set on the scroll layer (not the scrollbar
+  // layers) and consumed by LayerTreeImpl::PushPropertiesTo during activation.
+  bool needs_show_scrollbars_ : 1;
+
   DISALLOW_COPY_AND_ASSIGN(LayerImpl);
 };