Fixing Canvas2DLayerBridge to handle lost graphics contexts

Source/core/platform/graphics/chromium/Canvas2DLayerBridge.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
@@ skipping 69 matching lines @@
     virtual void flush(); // virtual for mocking
     virtual size_t storageAllocatedForRecording(); // virtual for faking
     size_t bytesAllocated() const {return m_bytesAllocated;}
     void limitPendingFrames();
 
     WebKit::WebLayer* layer();
     void contextAcquired();
 
     unsigned backBufferTexture();
 
+    bool contextIsValid();
+
 protected:
     Canvas2DLayerBridge(PassRefPtr<GraphicsContext3D>, SkDeferredCanvas*, OpacityMode, ThreadMode);
 
     SkDeferredCanvas* m_canvas;
     OwnPtr<WebKit::WebExternalTextureLayer> m_layer;
     RefPtr<GraphicsContext3D> m_context;
     size_t m_bytesAllocated;
     bool m_didRecordDrawCommand;
     int m_framesPending;
 
@@ skipping 19 matching lines @@
     MailboxInfo* createMailboxInfo();
 
     uint32_t m_lastImageId;
     Vector<MailboxInfo> m_mailboxes;
 #endif
 };
 
 }
 
 #endif