Roll ffmpeg to r120671

TBR=scherkus

Review URL: http://codereview.chromium.org/9347005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@120716 0039d316-1c4b-4281-b951-d872f2087c98

DEPS

@@ -34,7 +34,7 @@ vars = {
   "libphonenumber_revision": "407",
   "libvpx_revision": "119942",
   "lss_revision": "9",
-  "ffmpeg_revision": "120156",
+  "ffmpeg_revision": "120671",
   "sfntly_revision": "118",
   "skia_revision": "3138",
   # Three lines of non-changing comments so that