Web Animations: Remove scheduling logic that was merged incorrectly

A bad merge when landing r160394 meant that we were still constantly requesting
frame callbacks.

BUG=258896

Review URL: https://codereview.chromium.org/39953002

git-svn-id: svn://svn.chromium.org/blink/trunk@161756 bbb929c8-8fbe-4397-9dbb-9b2b20218538