Bug 1199032: [MSE/webm] P2. Construct WebMDemuxer with special MSE handling. r=kinetik

dom/media/mediasource/TrackBuffersManager.cpp

@@ -785,7 +785,7 @@ TrackBuffersManager::CreateDemuxerforMIMEType()
 
 #ifdef MOZ_WEBM
   if (mType.LowerCaseEqualsLiteral("video/webm") || mType.LowerCaseEqualsLiteral("audio/webm")) {
-    mInputDemuxer = new WebMDemuxer(mCurrentInputBuffer);
+    mInputDemuxer = new WebMDemuxer(mCurrentInputBuffer, true /* IsMediaSource*/ );
     return;
   }
 #endif