bug 1123189 Queue "durationchange" instead of dispatching synchronously r=cpearce

http://www.w3.org/TR/2014/REC-html5-20141028/embedded-content-0.html#offsets-into-the-media-resource

"When the length of the media resource changes to a known value (e.g. from
being unknown to known, or from a previously established length to a new
length) the user agent must queue a task to fire a simple event named
durationchange at the media element."

dom/html/HTMLMediaElement.cpp

@@ -204,7 +204,7 @@ protected:
   uint32_t mLoadID;
 };
 
-class nsAsyncEventRunner : public nsMediaEvent
+class HTMLMediaElement::nsAsyncEventRunner : public nsMediaEvent
 {
 private:
   nsString mName;