Bug 1396629 - P3. Make member const. r=cpearce

MozReview-Commit-ID: CwOh3BZj0vG

--HG--
extra : rebase_source : 698bdb76c3c713bd956448f0fc15eeeeab015b0e

dom/media/mediasource/TrackBuffersManager.h

@@ -211,7 +211,7 @@ private:
   // Set to true once a new segment is started.
   bool mNewMediaSegmentStarted;
   bool mActiveTrack;
-  MediaContainerType mType;
+  const MediaContainerType mType;
 
   // ContainerParser objects and methods.
   // Those are used to parse the incoming input buffer.