Dont drop packet when last_dts_ is invalid

If the previous packet position and the current match and we fall back
to comparing display timestamps, make sure that we don't drop packets
which never had a display timestamp set.

Bug: 894776
Change-Id: I5033462642255743cb05cd44ec85312441503f53
Reviewed-on: https://chromium-review.googlesource.com/c/1356310Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: Ted Meyer <tmathmeyer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612745}

media/filters/ffmpeg_demuxer.cc

@@ -391,8 +391,8 @@ void FFmpegDemuxerStream::EnqueuePacket(ScopedAVPacket packet) {
                << " < " << last_packet_pos_ << ")";
       return;
     }
-    if (packet->pos == last_packet_pos_ && packet_dts <= last_packet_dts_) {
-      DCHECK_NE(last_packet_dts_, AV_NOPTS_VALUE);
+    if (last_packet_dts_ != AV_NOPTS_VALUE && packet->pos == last_packet_pos_ &&
+        packet_dts <= last_packet_dts_) {
       DVLOG(3) << "Dropped packet with out of order display timestamp ("
                << packet_dts << " < " << last_packet_dts_ << ")";
       return;