Disable overlay scrollbars in Blink when hidden by the compositor.

cc/trees/layer_tree_host_impl.h

Index: cc/trees/layer_tree_host_impl.h
diff --git a/cc/trees/layer_tree_host_impl.h b/cc/trees/layer_tree_host_impl.h
index e0539a66a9795e96a95a458d82c763cae76d963e..b5eb7d23bd64e081ffd1fc16fe3dca4917178531 100644
--- a/cc/trees/layer_tree_host_impl.h
+++ b/cc/trees/layer_tree_host_impl.h
@@ -356,6 +356,7 @@ class CC_EXPORT LayerTreeHostImpl
   void SetNeedsAnimateForScrollbarAnimation() override;
   void SetNeedsRedrawForScrollbarAnimation() override;
   ScrollbarSet ScrollbarsFor(int scroll_layer_id) const override;
+  void SetScrollbarsHidden(int layer_id, bool hidden) override;
 
   // VideoBeginFrameSource implementation.
   void AddVideoFrameController(VideoFrameController* controller) override;
@@ -806,6 +807,11 @@ class CC_EXPORT LayerTreeHostImpl
   std::unordered_map<int, std::unique_ptr<ScrollbarAnimationController>>
       scrollbar_animation_controllers_;
 
+  // A map of scroll layers whose scrollbars have been hidden or shown since
+  // the last commit. This gets cleared every commit so only changes are sent
+  // back to the main thread.
+  std::unordered_map<int, bool> scrollbars_hidden_map_;
+
   RenderingStatsInstrumentation* rendering_stats_instrumentation_;
   MicroBenchmarkControllerImpl micro_benchmark_controller_;
   std::unique_ptr<SynchronousTaskGraphRunner>