Don't uninitialize FFmpegVideoDecodeEngine if we haven't initialized it.

This was originally commited as r70077 but was accidentally reverted.

BUG=67303
TEST=media_unittests

Review URL: http://codereview.chromium.org/6046006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@70669 0039d316-1c4b-4281-b951-d872f2087c98
3 files changed