Media: Reject negative initial timestamps in FFmpegDemuxer.

During recent changes, it became possible for the first video buffer
timestamp in a stream to be negative. This change restores the invariant
that video buffer timestampes are non-negative, by emitting a demuxer
error in this case.

BUG=710845

Review-Url: https://codereview.chromium.org/2822063002
Cr-Commit-Position: refs/heads/master@{#465678}

media/filters/ffmpeg_demuxer.cc

@@ -507,7 +507,8 @@ void FFmpegDemuxerStream::EnqueuePacket(ScopedAVPacket packet) {
 
   // Only allow negative timestamps past if we know they'll be fixed up by the
   // code paths below; otherwise they should be treated as a parse error.
-  if (!fixup_negative_timestamps_ && buffer->timestamp() < base::TimeDelta()) {
+  if ((!fixup_negative_timestamps_ || last_packet_timestamp_ == kNoTimestamp) &&
+      buffer->timestamp() < base::TimeDelta()) {
     MEDIA_LOG(DEBUG, media_log_)
         << "FFmpegDemuxer: unfixable negative timestamp";
     demuxer_->NotifyDemuxerError(DEMUXER_ERROR_COULD_NOT_PARSE);