Bug 1293646: [MSE] P1. Reject seeking attempt with EOS when the mediasource is ended. r=gerald

Otherwise, the ended event would never be fired should the decoder have reached the end of the stream prior endOfStream being called.

MozReview-Commit-ID: CbWCnzi3nxj

--HG--
extra : rebase_source : 729e25919fdb7f8a8918c4d5a9bcae17d8c8bdc5

dom/media/mediasource/MediaSourceDemuxer.cpp

@@ -387,8 +387,9 @@ MediaSourceTrackDemuxer::DoSeek(media::TimeUnit aTime)
   if (!buffered.Contains(seekTime)) {
     if (!buffered.Contains(aTime)) {
       // We don't have the data to seek to.
-      return SeekPromise::CreateAndReject(DemuxerFailureReason::WAITING_FOR_DATA,
-                                          __func__);
+      return SeekPromise::CreateAndReject(
+        mManager->IsEnded() ? DemuxerFailureReason::END_OF_STREAM :
+                              DemuxerFailureReason::WAITING_FOR_DATA, __func__);
     }
     // Theorically we should reject the promise with WAITING_FOR_DATA,
     // however, to avoid unwanted regressions we assume that if at this time