Roll FFmpeg DEPS to pull in mp3 decoding fixes.

Pulls in:
ffa61fc mp3: fix start band index for block type 2 in 8kHz audio
9894d3a mpegaudidec: 10l forgot buf_size
ed6e594 mpegaudiodec: skip initial zeros.
555537f mp3demux: fix id3 discard code
5ecdf34 mp3demux: fix off by 1 error
598bd74 mp3decoder: discard ID3 tags
0c706ac Mark truncated packets as corrupt in av_get_packet.

BUG=69730,128826
TEST=mp3 files in bugs. media_unittests. ffmpeg_regression_tests.
TBR=scherkus

Review URL: https://chromiumcodereview.appspot.com/10399110

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@138138 0039d316-1c4b-4281-b951-d872f2087c98

DEPS

@@ -51,8 +51,8 @@ vars = {
 
   # These two FFmpeg variables must be updated together.  One is used for SVN
   # checkouts and the other for Git checkouts.
-  "ffmpeg_revision": "137006",
-  "ffmpeg_hash": "4ece657f9dad6a37e7b7e3584eb8887aad4dc499",
+  "ffmpeg_revision": "138127",
+  "ffmpeg_hash": "9b8f7bea0a9b919b15a53e1bad56f251e41b558b",
 
   "sfntly_revision": "128",
   "skia_revision": "4012",