Added an assertion verifying that a Canvas2DLayerBridge is not deleted

Source/platform/graphics/Canvas2DLayerManager.h

 /*
 Copyright (C) 2012 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.
 2.  Redistributions in binary form must reproduce the above copyright
     notice, this list of conditions and the following disclaimer in the
@@ skipping 24 matching lines @@
 class PLATFORM_EXPORT Canvas2DLayerManager : public blink::WebThread::TaskObserver {
 public:
     static Canvas2DLayerManager& get();
 
     void init(size_t maxBytesAllocated, size_t targetBytesAllocated);
     virtual ~Canvas2DLayerManager();
 
     void layerTransientResourceAllocationChanged(Canvas2DLayerBridge*, intptr_t deltaBytes = 0);
     void layerDidDraw(Canvas2DLayerBridge*);
 
+    bool isInList(Canvas2DLayerBridge*) const;
 private:
     Canvas2DLayerManager();
 
     // internal methods
     void freeMemoryIfNecessary();
-    bool isInList(Canvas2DLayerBridge*) const;
     void addLayerToList(Canvas2DLayerBridge*);
     void removeLayerFromList(Canvas2DLayerBridge*);
     virtual void willProcessTask() OVERRIDE;
     virtual void didProcessTask() OVERRIDE;
 
     size_t m_bytesAllocated;
     size_t m_maxBytesAllocated;
     size_t m_targetBytesAllocated;
     bool m_taskObserverActive;
     DoublyLinkedList<Canvas2DLayerBridge> m_layerList;
 
     friend class ::Canvas2DLayerManagerTest; // for unit testing
 };
 
 }
 
 #endif