Oilpan: move MediaKeyError to the heap.

Source/core/html/MediaKeyEvent.h

Index: Source/core/html/MediaKeyEvent.h
diff --git a/Source/core/html/MediaKeyEvent.h b/Source/core/html/MediaKeyEvent.h
index cf95046f6574dde49fe08425cc7a52efebf086b2..404f043b6594ce985819f5fddb6907c0930b8148 100644
--- a/Source/core/html/MediaKeyEvent.h
+++ b/Source/core/html/MediaKeyEvent.h
@@ -39,7 +39,7 @@ struct MediaKeyEventInit : public EventInit {
     RefPtr<Uint8Array> initData;
     RefPtr<Uint8Array> message;
     String defaultURL;
-    RefPtr<MediaKeyError> errorCode;
+    RefPtrWillBeMember<MediaKeyError> errorCode;
     unsigned short systemCode;
 };
 
@@ -79,7 +79,7 @@ private:
     RefPtr<Uint8Array> m_initData;
     RefPtr<Uint8Array> m_message;
     String m_defaultURL;
-    RefPtr<MediaKeyError> m_errorCode;
+    RefPtrWillBeMember<MediaKeyError> m_errorCode;
     unsigned short m_systemCode;
 };