Disable overlay scrollbars in Blink when hidden by the compositor.

cc/trees/property_tree.h

Index: cc/trees/property_tree.h
diff --git a/cc/trees/property_tree.h b/cc/trees/property_tree.h
index ce6b618fc528108dc2781cf4981937b672f0c20f..a83d38ef530992c0c815e63084d096242c6cb3dd 100644
--- a/cc/trees/property_tree.h
+++ b/cc/trees/property_tree.h
@@ -422,6 +422,7 @@ class CC_EXPORT ScrollTree final : public PropertyTree<ScrollNode> {
 
   typedef std::unordered_map<int, scoped_refptr<SyncedScrollOffset>>
       ScrollOffsetMap;
+  typedef std::unordered_map<int, bool> ScrollbarsEnabledMap;
 
   gfx::ScrollOffset MaxScrollOffset(int scroll_node_id) const;
   void OnScrollOffsetAnimated(int layer_id,
@@ -470,6 +471,7 @@ class CC_EXPORT ScrollTree final : public PropertyTree<ScrollNode> {
  private:
   int currently_scrolling_node_id_;
   ScrollOffsetMap layer_id_to_scroll_offset_map_;
+  ScrollbarsEnabledMap layer_id_to_scrollbars_enabled_map_;
 
   SyncedScrollOffset* synced_scroll_offset(int layer_id);
   const SyncedScrollOffset* synced_scroll_offset(int layer_id) const;