Roll FFmpeg DEPS.

Pulls in the following patches:
172513a Linux: upstream Gentoo Linux patches:
cfe978e Remove trailing periods from rule messages.

BUG=none
TEST=none
TBR=thakis

Review URL: https://codereview.chromium.org/134333004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@244326 0039d316-1c4b-4281-b951-d872f2087c98

DEPS

@@ -39,8 +39,8 @@ vars = {
 
   # These two FFmpeg variables must be updated together.  One is used for SVN
   # checkouts and the other for Git checkouts.
-  "ffmpeg_revision": "242831",
-  "ffmpeg_hash": "0b461999804771144964b14bd86a0be885ff7d48",
+  "ffmpeg_revision": "244257",
+  "ffmpeg_hash": "172513a0692e299fb82e95f823351eb3696452d6",
 
   "sfntly_revision": "228",
   "lighttpd_revision": "33737",