Disable overlay scrollbars in Blink when hidden by the compositor.

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

 /*
  * Copyright (C) 2009 Apple Inc. 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
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 1145 matching lines @@
   tracedValue->SetString(
       "layer_name", WTF::StringUTF8Adaptor(debugName(layer)).asStringPiece());
   return std::move(tracedValue);
 }
 
 void GraphicsLayer::didUpdateMainThreadScrollingReasons() {
   m_debugInfo.setMainThreadScrollingReasons(
       platformLayer()->mainThreadScrollingReasons());
 }
 
+void GraphicsLayer::didChangeScrollbarsHidden(bool hidden) {
+  if (m_scrollableArea)
+    m_scrollableArea->setScrollbarsHidden(hidden);
+}
+
 PaintController& GraphicsLayer::getPaintController() {
   RELEASE_ASSERT(drawsContent());
   if (!m_paintController)
     m_paintController = PaintController::create();
   return *m_paintController;
 }
 
 void GraphicsLayer::setElementId(const CompositorElementId& id) {
   if (WebLayer* layer = platformLayer())
     layer->setElementId(id);
@@ skipping 124 matching lines @@
 void showGraphicsLayerTree(const blink::GraphicsLayer* layer) {
   if (!layer) {
     fprintf(stderr, "Cannot showGraphicsLayerTree for (nil).\n");
     return;
   }
 
   String output = layer->layerTreeAsText(blink::LayerTreeIncludesDebugInfo);
   fprintf(stderr, "%s\n", output.utf8().data());
 }
 #endif