Fix imageSourceURL() for WebGL elements.

Source/core/html/canvas/WebGLRenderingContextBase.h

Index: Source/core/html/canvas/WebGLRenderingContextBase.h
diff --git a/Source/core/html/canvas/WebGLRenderingContextBase.h b/Source/core/html/canvas/WebGLRenderingContextBase.h
index d80e30431d449110d4e4a36b627384398117c9e3..ca08a621efc166b289a7326af6a861c2fd9a195b 100644
--- a/Source/core/html/canvas/WebGLRenderingContextBase.h
+++ b/Source/core/html/canvas/WebGLRenderingContextBase.h
@@ -348,6 +348,7 @@ public:
     virtual bool hasPendingActivity() const OVERRIDE;
     virtual void stop() OVERRIDE;
 
+    void setSavingImage(bool isSaving) { m_savingImage = isSaving; }
 protected:
     friend class WebGLDrawBuffers;
     friend class WebGLFramebuffer;
@@ -535,6 +536,8 @@ protected:
 
     OwnPtr<Extensions3DUtil> m_extensionsUtil;
 
+    bool m_savingImage;
+
     enum ExtensionFlags {
         ApprovedExtension               = 0x00,
         // Extension that is behind the draft extensions runtime flag: