Disable overlay scrollbars in Blink when hidden by the compositor.

ui/compositor/layer.h

 // Copyright (c) 2012 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 UI_COMPOSITOR_LAYER_H_
 #define UI_COMPOSITOR_LAYER_H_
 
 #include <stddef.h>
 
 #include <memory>
@@ skipping 355 matching lines @@
   bool PrepareTextureMailbox(
       cc::TextureMailbox* mailbox,
       std::unique_ptr<cc::SingleReleaseCallback>* release_callback) override;
 
   float device_scale_factor() const { return device_scale_factor_; }
 
   // LayerClient
   std::unique_ptr<base::trace_event::ConvertableToTraceFormat> TakeDebugInfo(
       cc::Layer* layer) override;
   void didUpdateMainThreadScrollingReasons() override;
+  void didChangeScrollbarsHidden(bool) override;
 
   // Triggers a call to SwitchToLayer.
   void SwitchCCLayerForTest();
 
   const cc::Region& damaged_region_for_testing() const {
     return damaged_region_;
   }
 
  private:
   friend class LayerOwner;
@@ skipping 136 matching lines @@
   // The size of the frame or texture in DIP, set when SetShowDelegatedContent
   // or SetTextureMailbox was called.
   gfx::Size frame_size_in_dip_;
 
   DISALLOW_COPY_AND_ASSIGN(Layer);
 };
 
 }  // namespace ui
 
 #endif  // UI_COMPOSITOR_LAYER_H_