Fixing Canvas2DLayerBridge to handle lost graphics contexts

Source/core/platform/graphics/ImageBuffer.h

Index: Source/core/platform/graphics/ImageBuffer.h
diff --git a/Source/core/platform/graphics/ImageBuffer.h b/Source/core/platform/graphics/ImageBuffer.h
index bbbb183a497f5e238647f584c5fac498956d68d0..e0a2a3048f2c020e15f58fbeaf44b3d13e6d16aa 100644
--- a/Source/core/platform/graphics/ImageBuffer.h
+++ b/Source/core/platform/graphics/ImageBuffer.h
@@ -131,6 +131,8 @@ public:
     bool copyToPlatformTexture(GraphicsContext3D&, Platform3DObject, GC3Denum, GC3Denum, GC3Dint, bool, bool);
 
 private:
+    bool isValid() const;
+
     void draw(GraphicsContext*, const FloatRect&, const FloatRect& = FloatRect(0, 0, -1, -1), CompositeOperator = CompositeSourceOver, BlendMode = BlendModeNormal, bool useLowQualityScale = false);
     void drawPattern(GraphicsContext*, const FloatRect&, const FloatSize&, const FloatPoint&, CompositeOperator, const FloatRect&);