Add av_stream_get_first_dts for Chromium

Index: libavformat/avformat.h
--- libavformat/avformat.h.orig
+++ libavformat/avformat.h
@@ -1030,6 +1030,10 @@ attribute_deprecated
 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);
+// Chromium: We use the internal field first_dts ^^^
+
 #define AV_PROGRAM_RUNNING 1
 
 /**
