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") {
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<div class='path'>path: <a href='/guix/log/?id=8d3167fe2399656d62d72c280b6a343c5f64a3b4&amp;showmsg=1'>root</a>/<a href='/guix/log/CODE-OF-CONDUCT?id=8d3167fe2399656d62d72c280b6a343c5f64a3b4&amp;showmsg=1'>CODE-OF-CONDUCT</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/CODE-OF-CONDUCT?id=8d3167fe2399656d62d72c280b6a343c5f64a3b4'>Collapse</a>)</th><th class='left'>Author</th></tr>
<tr class='logheader'><td><span title='2018-05-29 18:29:36 +0200'>2018-05-29</span></td><td class='logsubject'><a href='/guix/commit/CODE-OF-CONDUCT?id=a076f19908d06b6df49f1c25c40de8838213cd71'>CODE-OF-CONDUCT: Switch to version 1.4 of the Contributor Covenant.</a></td><td>Ludovic Courtès</td></tr>
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See the discussion at
&lt;https://lists.gnu.org/archive/html/guix-devel/2018-05/msg00146.html&gt;.

* CODE-OF-CONDUCT: Adapt from version 1.4 of the Contributor Covenant.


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