Disable overlay scrollbars in Blink when hidden by the compositor.

third_party/WebKit/Source/platform/graphics/GraphicsLayer.h

 /*
  * Copyright (C) 2009 Apple Inc. All rights reserved.
  * Copyright (C) 2013 Intel Corporation. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 243 matching lines @@
   void paint(const IntRect* interestRect,
              GraphicsContext::DisabledMode = GraphicsContext::NothingDisabled);
 
   // WebLayerScrollClient implementation.
   void didScroll() override;
 
   // cc::LayerClient implementation.
   std::unique_ptr<base::trace_event::ConvertableToTraceFormat> TakeDebugInfo(
       cc::Layer*) override;
   void didUpdateMainThreadScrollingReasons() override;
+  void didChangeScrollbarsHidden(bool);
 
   PaintController& getPaintController();
 
   // Exposed for tests.
   WebLayer* contentsLayer() const { return m_contentsLayer; }
 
   void setElementId(const CompositorElementId&);
   CompositorElementId elementId() const;
 
   void setCompositorMutableProperties(uint32_t);
@@ skipping 134 matching lines @@
 };
 
 }  // namespace blink
 
 #ifndef NDEBUG
 // Outside the blink namespace for ease of invocation from gdb.
 void PLATFORM_EXPORT showGraphicsLayerTree(const blink::GraphicsLayer*);
 #endif
 
 #endif  // GraphicsLayer_h