Add av_stream_get_first_dts for Chromium

Index: libavformat/mux_utils.c
--- libavformat/mux_utils.c.orig
+++ libavformat/mux_utils.c
@@ -40,6 +40,13 @@ int64_t av_stream_get_end_pts(const AVStream *st)
 }
 #endif
 
+// Chromium: We use the internal field first_dts vvv
+int64_t av_stream_get_first_dts(const AVStream *st)
+{
+  return cffstream(st)->first_dts;
+}
+// Chromium: We use the internal field first_dts ^^^
+
 int avformat_query_codec(const AVOutputFormat *ofmt, enum AVCodecID codec_id,
                          int std_compliance)
 {
