Bug 939716 - Destructor for MediaStreamGraph should be virtual - r=roc

The destructor for MediaStreamGraph is non-virtual but it has a
derived class, MediaStreamGraphImpl, that overrides the destructor.
This destructor won't be called if an instance is deleted using a
pointer type of MediaStreamGraph.

It's unclear whether this actually happens but MediaStreamGraph.cpp
has code that creates a MediaStreamGraphImpl and returns it as a
MediaStreamGraph.

--HG--
extra : rebase_source : 9d006e5bf9bb056b5a54fc7aec159c26376c1459

content/media/MediaStreamGraph.h

@@ -1057,7 +1057,7 @@ protected:
   {
     MOZ_COUNT_CTOR(MediaStreamGraph);
   }
-  ~MediaStreamGraph()
+  virtual ~MediaStreamGraph()
   {
     MOZ_COUNT_DTOR(MediaStreamGraph);
   }

content/media/MediaStreamGraphImpl.h

@@ -113,7 +113,7 @@ public:
    * implement OfflineAudioContext.  They do not support MediaStream inputs.
    */
   explicit MediaStreamGraphImpl(bool aRealtime);
-  ~MediaStreamGraphImpl();
+  virtual ~MediaStreamGraphImpl();
 
   // Main thread only.
   /**