Add /chrome/test/media/license_server/widevine/ to .gitignore.

(Otherwise this shows up in git status if you have a checkout with
src-internal.)

R=maruel@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/148733002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247478 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247478