Make WebGL extensions get lost when context is lost.

Source/core/html/canvas/WebGLExtension.h

Index: Source/core/html/canvas/WebGLExtension.h
diff --git a/Source/core/html/canvas/WebGLExtension.h b/Source/core/html/canvas/WebGLExtension.h
index 84eef1bcd1fc2882fe781208c3effc86271f775d..a27cc3c79c3dfb430d6c7fc1da264da6b0f407a6 100644
--- a/Source/core/html/canvas/WebGLExtension.h
+++ b/Source/core/html/canvas/WebGLExtension.h
@@ -27,10 +27,11 @@
 #define WebGLExtension_h
 
 #include "core/html/canvas/WebGLRenderingContext.h"
+#include "wtf/RefCounted.h"
 
 namespace WebCore {
 
-class WebGLExtension {
+class WebGLExtension : public RefCounted<WebGLExtension> {
     WTF_MAKE_FAST_ALLOCATED;
 public:
     // Extension names are needed to properly wrap instances in JavaScript objects.
@@ -54,15 +55,24 @@ public:
         WebGLLoseContextName,
     };
 
-    void ref() { m_context->ref(); }
-    void deref() { m_context->deref(); }
     WebGLRenderingContext* context() { return m_context; }
 
     virtual ~WebGLExtension();
     virtual ExtensionName getName() const = 0;
 
+    virtual void lose()
+    {
+        m_context = 0;
+    }
+
+    bool isLost()
+    {
+        return !m_context;
+    }
+
 protected:
     WebGLExtension(WebGLRenderingContext*);
+
     WebGLRenderingContext* m_context;
 };