Bug 1148102: P1. Mark WebMBufferedParser refcounting as thread-safe. r=kinetik

This is required should we use WebMBufferedParser in a MediaTaskQueue as we don't know which actual thread will actually be used.

dom/media/webm/WebMBufferedParser.h

@@ -221,7 +221,7 @@ private:
 
 class WebMBufferedState final
 {
-  NS_INLINE_DECL_REFCOUNTING(WebMBufferedState)
+  NS_INLINE_DECL_THREADSAFE_REFCOUNTING(WebMBufferedState)
 
 public:
   WebMBufferedState() : mReentrantMonitor("WebMBufferedState") {