Bug 1411694 - add an override declaration in WebrtcMediaCodecVP8VideoCodec; r=jesup

We have override declarations for lots of other overriden virtual
functions, but not this one.

media/webrtc/signaling/src/media-conduit/WebrtcMediaCodecVP8VideoCodec.cpp

@@ -92,7 +92,7 @@ public:
     CSFLogDebug(LOGTAG,  "%s %p", __FUNCTION__, this);
   }
 
-  void HandleOutput(Sample::Param aSample)
+  void HandleOutput(Sample::Param aSample) override
   {
     CSFLogDebug(LOGTAG,  "%s %p", __FUNCTION__, this);
     BufferInfo::LocalRef info = aSample->Info();