UseCounter: Introduce UseCounter::Observer for layout tests

third_party/WebKit/Source/core/page/Page.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013 Apple Inc. All
  * Rights Reserved.
  * Copyright (C) 2008 Torch Mobile Inc. All rights reserved.
  * (http://www.torchmobile.com/)
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
@@ skipping 482 matching lines @@
   visitor->trace(m_autoscrollController);
   visitor->trace(m_chromeClient);
   visitor->trace(m_dragCaret);
   visitor->trace(m_dragController);
   visitor->trace(m_focusController);
   visitor->trace(m_contextMenuController);
   visitor->trace(m_pointerLockController);
   visitor->trace(m_scrollingCoordinator);
   visitor->trace(m_mainFrame);
   visitor->trace(m_validationMessageClient);
+  visitor->trace(m_useCounter);
   visitor->trace(m_frameHost);
   Supplementable<Page>::trace(visitor);
   PageVisibilityNotifier::trace(visitor);
 }
 
 void Page::layerTreeViewInitialized(WebLayerTreeView& layerTreeView,
                                     FrameView* view) {
   if (scrollingCoordinator())
     scrollingCoordinator()->layerTreeViewInitialized(layerTreeView, view);
 }
@@ skipping 28 matching lines @@
     : chromeClient(nullptr),
       contextMenuClient(nullptr),
       editorClient(nullptr),
       spellCheckerClient(nullptr) {}
 
 Page::PageClients::~PageClients() {}
 
 template class CORE_TEMPLATE_EXPORT Supplement<Page>;
 
 }  // namespace blink