Make WebGL extensions get lost when context is lost.

Source/core/html/canvas/OESStandardDerivatives.h

Index: Source/core/html/canvas/OESStandardDerivatives.h
diff --git a/Source/core/html/canvas/OESStandardDerivatives.h b/Source/core/html/canvas/OESStandardDerivatives.h
index d2d2ce20f85dd2576e429b00f7de5e2cc7b35756..eee4ab55e282baea2ae5c4fd7fb73206d2cb6f08 100644
--- a/Source/core/html/canvas/OESStandardDerivatives.h
+++ b/Source/core/html/canvas/OESStandardDerivatives.h
@@ -28,13 +28,13 @@
 
 #include "bindings/v8/ScriptWrappable.h"
 #include "core/html/canvas/WebGLExtension.h"
-#include "wtf/PassOwnPtr.h"
+#include "wtf/PassRefPtr.h"
 
 namespace WebCore {
 
 class OESStandardDerivatives : public WebGLExtension, public ScriptWrappable {
 public:
-    static PassOwnPtr<OESStandardDerivatives> create(WebGLRenderingContext*);
+    static PassRefPtr<OESStandardDerivatives> create(WebGLRenderingContext*);
     static bool supported(WebGLRenderingContext*);
     static const char* getExtensionName();