Disable overlay scrollbars in Blink when hidden by the compositor.

cc/layers/layer.h

Index: cc/layers/layer.h
diff --git a/cc/layers/layer.h b/cc/layers/layer.h
index 5f98c71f480001db300392e3eb4bc8a572ad95c8..5f9f4865d0fa8500ae278afff21858083458ad47 100644
--- a/cc/layers/layer.h
+++ b/cc/layers/layer.h
@@ -500,6 +500,8 @@ class CC_EXPORT Layer : public base::RefCounted<Layer> {
   // layer id for the corresponding Layer on the engine.
   void SetLayerIdForTesting(int id);
 
+  void SetScrollbarsHiddenFromImplSide(bool hidden);
+
  protected:
   friend class LayerImpl;
   friend class TreeSynchronizer;
@@ -721,6 +723,7 @@ class CC_EXPORT Layer : public base::RefCounted<Layer> {
   bool force_render_surface_for_testing_ : 1;
   bool subtree_property_changed_ : 1;
   bool may_contain_video_ : 1;
+  bool scrollbars_hidden_ : 1;
   SkColor safe_opaque_background_color_;
   // draw_blend_mode may be different than blend_mode_,
   // when a RenderSurface re-parents the layer's blend_mode.