Bug 1247189 - part5 : use IsCurrentThreadIn() as a main criteria to determine whether we're on the task queue or not. r=jya

The use of the TrackBuffersManager once detached is explictly forbidden, as such
OnTaskQueue() can only be used before the DetachTask ran: we now strongly assert
as such.

MozReview-Commit-ID: ycOI4QRElb

--HG--
extra : rebase_source : 44ea3d0eb292e5c285d0aa4e10eefa41f20beed7

dom/media/mediasource/TrackBuffersManager.h

@@ -461,7 +461,8 @@ private:
   }
   bool OnTaskQueue() const
   {
-    return !GetTaskQueue() || GetTaskQueue()->IsCurrentThreadIn();
+    MOZ_RELEASE_ASSERT(GetTaskQueue());
+    return GetTaskQueue()->IsCurrentThreadIn();
   }
   RefPtr<AutoTaskQueue> mTaskQueue;