Alias the av_stream_get_first_dts definition added by Chromium in
their bundled fork.

Taken from Arch Linux:

  https://github.com/archlinux/svntogit-packages/blob/38fd5d2f2e0ade932aa7a80b7dea26215fc41854/trunk/unbundle-ffmpeg-av_stream_get_first_dts.patch

diff --git a/build/linux/unbundle/ffmpeg.gn b/build/linux/unbundle/ffmpeg.gn
--- a/build/linux/unbundle/ffmpeg.gn
+++ b/build/linux/unbundle/ffmpeg.gn
@@ -12,6 +12,7 @@ pkg_config("system_ffmpeg") {
     "libavformat",
     "libavutil",
   ]
+  defines = [ "av_stream_get_first_dts(stream)=stream->first_dts" ]
 }
 
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