Adding mailing list for animation code reviews.

The new group has been created at
https://groups.google.com/a/chromium.org/forum/#!forum/blink-reviews-animation

Review URL: https://codereview.chromium.org/264043006

git-svn-id: svn://svn.chromium.org/blink/trunk@173357 bbb929c8-8fbe-4397-9dbb-9b2b20218538