Bug 1371200. P5 - devirtualize some functions that don't have overrides. r=cpearce

MozReview-Commit-ID: CtWPXF6MYIi

--HG--
extra : rebase_source : c0d147cb878ddf2147355c02419fd7d7b2044a40
extra : source : 6467bf865cdcefd5b41049045ac9d34b101d5673

dom/media/MediaDecoder.h

@@ -223,9 +223,9 @@ public:
   // Pause video playback.
   virtual void Pause();
   // Adjust the speed of the playback, optionally with pitch correction,
-  virtual void SetVolume(double aVolume);
+  void SetVolume(double aVolume);
 
-  virtual void SetPlaybackRate(double aPlaybackRate);
+  void SetPlaybackRate(double aPlaybackRate);
   void SetPreservesPitch(bool aPreservesPitch);
 
   bool GetMinimizePreroll() const { return mMinimizePreroll; }