Fixing crashes in layout tests when virtual-viewport is enabled.

Source/core/inspector/InspectorLayerTreeAgent.h

 /*
  * Copyright (C) 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2013 Google 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.
@@ skipping 70 matching lines @@
     virtual void snapshotCommandLog(ErrorString*, const String& snapshotId, RefPtr<TypeBuilder::Array<JSONObject> >&) override;
 
     // Called by other agents.
     PassRefPtr<TypeBuilder::Array<TypeBuilder::LayerTree::Layer> > buildLayerTree();
 
 private:
     static unsigned s_lastSnapshotId;
 
     explicit InspectorLayerTreeAgent(Page*);
 
+    GraphicsLayer* rootGraphicsLayer();
+
     RenderLayerCompositor* renderLayerCompositor();
     GraphicsLayer* layerById(ErrorString*, const String& layerId);
     const GraphicsContextSnapshot* snapshotById(ErrorString*, const String& snapshotId);
 
     typedef HashMap<int, int> LayerIdToNodeIdMap;
     void buildLayerIdToNodeIdMap(RenderLayer*, LayerIdToNodeIdMap&);
     void gatherGraphicsLayers(GraphicsLayer*, HashMap<int, int>& layerIdToNodeIdMap, RefPtr<TypeBuilder::Array<TypeBuilder::LayerTree::Layer> >&);
     int idForNode(Node*);
 
     InspectorFrontend::LayerTree* m_frontend;
     RawPtrWillBeMember<Page> m_page;
     Vector<int, 2> m_pageOverlayLayerIds;
 
     typedef HashMap<String, RefPtr<GraphicsContextSnapshot> > SnapshotById;
     SnapshotById m_snapshotById;
 };
 
 } // namespace blink
 
 
 #endif // !defined(InspectorLayerTreeAgent_h)