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" ]
 }
 
 buildflag_header("ffmpeg_features") {
den' name='id' value='785b6590186759d8a9a9d231e7d8ff958707a808'/><select name='h' onchange='this.form.submit();'>
<option value='koszko' selected='selected'>koszko</option>
<option value='koszko-scripts'>koszko-scripts</option>
</select> <input type='submit' value='switch'/></form></td></tr>
<tr><td class='sub'>Wojtek's customized Guix</td><td class='sub right'></td></tr></table>
<table class='tabs'><tr><td>
<a href='/guix/about/'>about</a><a href='/guix/'>summary</a><a href='/guix/refs/?id=785b6590186759d8a9a9d231e7d8ff958707a808'>refs</a><a class='active' href='/guix/log/build-aux/check-channel-news.scm'>log</a><a href='/guix/tree/build-aux/check-channel-news.scm?id=785b6590186759d8a9a9d231e7d8ff958707a808'>tree</a><a href='/guix/commit/build-aux/check-channel-news.scm?id=785b6590186759d8a9a9d231e7d8ff958707a808'>commit</a><a href='/guix/diff/build-aux/check-channel-news.scm?id=785b6590186759d8a9a9d231e7d8ff958707a808'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/build-aux/check-channel-news.scm'>
<input type='hidden' name='id' value='785b6590186759d8a9a9d231e7d8ff958707a808'/><select name='qt'>
<option value='grep'>log msg</option>
<option value='author'>author</option>
<option value='committer'>committer</option>
<option value='range'>range</option>
</select>
<input class='txt' type='search' size='10' name='q' value=''/>
<input type='submit' value='search'/>
</form>
</td></tr></table>
<div class='path'>path: <a href='/guix/log/?id=785b6590186759d8a9a9d231e7d8ff958707a808'>root</a>/<a href='/guix/log/build-aux?id=785b6590186759d8a9a9d231e7d8ff958707a808'>build-aux</a>/<a href='/guix/log/build-aux/check-channel-news.scm?id=785b6590186759d8a9a9d231e7d8ff958707a808'>check-channel-news.scm</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/build-aux/check-channel-news.scm?id=785b6590186759d8a9a9d231e7d8ff958707a808&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>