Bug 1172394 - Don't remove dom::MediaTracks on chained metadata updates. r=padenot

HTMLMediaElement avoid creating new tracks in MetadataLoaded when it has already
created some, so there should be no side effect to this patch.

Differential Revision: https://phabricator.services.mozilla.com/D52037

--HG--
extra : moz-landing-system : lando

dom/media/MediaDecoder.cpp

@@ -642,7 +642,6 @@ double MediaDecoder::GetCurrentTime() {
 void MediaDecoder::OnMetadataUpdate(TimedMetadata&& aMetadata) {
   MOZ_ASSERT(NS_IsMainThread());
   AbstractThread::AutoEnter context(AbstractMainThread());
-  GetOwner()->RemoveMediaTracks();
   MetadataLoaded(MakeUnique<MediaInfo>(*aMetadata.mInfo),
                  UniquePtr<MetadataTags>(std::move(aMetadata.mTags)),
                  MediaDecoderEventVisibility::Observable);