Update FFmpeg DEPS for security fix.

Pulls in:
fe7d549 Update README for security fix.
9d8789e vorbisdec: Check bark_map_size.
c81a7da Explicitly set cygwin_msvs_shell on

BUG=168473
TEST=unittests, manual.
TBR=scherkus


Review URL: https://chromiumcodereview.appspot.com/11889018

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@176861 0039d316-1c4b-4281-b951-d872f2087c98