Disable overlay scrollbars in Blink when hidden by the compositor.

cc/trees/layer_tree_host_impl.h

 // Copyright 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CC_TREES_LAYER_TREE_HOST_IMPL_H_
 #define CC_TREES_LAYER_TREE_HOST_IMPL_H_
 
 #include <stddef.h>
 
 #include <bitset>
@@ skipping 338 matching lines @@
       TreePriority tree_priority) override;
   void SetIsLikelyToRequireADraw(bool is_likely_to_require_a_draw) override;
   gfx::ColorSpace GetTileColorSpace() const override;
 
   // ScrollbarAnimationControllerClient implementation.
   void PostDelayedScrollbarAnimationTask(const base::Closure& task,
                                          base::TimeDelta delay) override;
   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;
   void RemoveVideoFrameController(VideoFrameController* controller) override;
 
   // CompositorFrameSinkClient implementation.
   void SetBeginFrameSource(BeginFrameSource* source) override;
   void SetExternalTilePriorityConstraints(
       const gfx::Rect& viewport_rect,
       const gfx::Transform& transform) override;
@@ skipping 430 matching lines @@
   gfx::Rect viewport_damage_rect_;
 
   std::unique_ptr<AnimationHost> animation_host_;
   std::set<VideoFrameController*> video_frame_controllers_;
 
   // Map from scroll layer ID to scrollbar animation controller.
   // There is one animation controller per pair of overlay scrollbars.
   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>
       single_thread_synchronous_task_graph_runner_;
 
   // Optional callback to notify of new tree activations.
   base::Closure tree_activation_callback_;
 
   SharedBitmapManager* shared_bitmap_manager_;
   gpu::GpuMemoryBufferManager* gpu_memory_buffer_manager_;
@@ skipping 15 matching lines @@
 
   std::unique_ptr<PendingTreeDurationHistogramTimer>
       pending_tree_duration_timer_;
 
   DISALLOW_COPY_AND_ASSIGN(LayerTreeHostImpl);
 };
 
 }  // namespace cc
 
 #endif  // CC_TREES_LAYER_TREE_HOST_IMPL_H_