Disable overlay scrollbars in Blink when hidden by the compositor.

cc/layers/layer_client.h

 // Copyright 2013 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_LAYERS_LAYER_CLIENT_H_
 #define CC_LAYERS_LAYER_CLIENT_H_
 
 #include <memory>
 
 #include "cc/base/cc_export.h"
@@ skipping 13 matching lines @@
   // Returns a pointer to a debug info object, if one has been computed.
   // If not, returns nullptr.
   // If the returned pointer is non-nullptr, the caller takes
   // ownership of the pointer.
   //
   // A pointer to the layer is provided for the convenience of layer clients
   // which service multiple layers.
   virtual std::unique_ptr<base::trace_event::ConvertableToTraceFormat>
   TakeDebugInfo(Layer* layer) = 0;
   virtual void didUpdateMainThreadScrollingReasons() = 0;
+  virtual void didChangeScrollbarsHidden(bool) = 0;
 
  protected:
   virtual ~LayerClient() {}
 };
 
 }  // namespace cc
 
 #endif  // CC_LAYERS_LAYER_CLIENT_H_