Fixing Canvas2DLayerBridge to handle lost graphics contexts

Source/core/platform/graphics/ImageBuffer.h

 /*
  * Copyright (C) 2006 Nikolas Zimmermann <zimmermann@kde.org>
  * Copyright (C) 2007, 2008, 2009 Apple Inc. All rights reserved.
  * Copyright (C) 2010 Torch Mobile (Beijing) Co. Ltd. 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 111 matching lines @@
         PlatformLayer* platformLayer() const;
 
         // FIXME: current implementations of this method have the restriction that they only work
         // with textures that are RGB or RGBA format, UNSIGNED_BYTE type and level 0, as specified in
         // Extensions3D::canUseCopyTextureCHROMIUM().
         bool copyToPlatformTexture(GraphicsContext3D&, Platform3DObject, GC3Denum, GC3Denum, GC3Dint, bool, bool);
 
         void reportMemoryUsage(MemoryObjectInfo*) const;
 
     private:
+        bool isValid() const;
         void clip(GraphicsContext*, const FloatRect&) const;
 
         void draw(GraphicsContext*, ColorSpace, const FloatRect& destRect, const FloatRect& srcRect = FloatRect(0, 0, -1, -1), CompositeOperator = CompositeSourceOver, BlendMode = BlendModeNormal, bool useLowQualityScale = false);
         void drawPattern(GraphicsContext*, const FloatRect& srcRect, const AffineTransform& patternTransform, const FloatPoint& phase, ColorSpace styleColorSpace, CompositeOperator, const FloatRect& destRect);
 
         inline void genericConvertToLuminanceMask();
 
         friend class GraphicsContext;
         friend class GeneratedImage;
         friend class CrossfadeGeneratedImage;
@@ skipping 10 matching lines @@
         // so that create() knows when it should return failure.
         ImageBuffer(const IntSize&, float resolutionScale, ColorSpace, RenderingMode, OpacityMode, bool& success);
         ImageBuffer(const IntSize&, float resolutionScale, ColorSpace, const GraphicsContext*, bool hasAlpha, bool& success);
     };
 
     String ImageDataToDataURL(const ImageData&, const String& mimeType, const double* quality);
 
 } // namespace WebCore
 
 #endif // ImageBuffer_h