Web Animations: Time to effect change should be infinity when player is held

This was causing frames to be requested continuously when animations were paused.

Review URL: https://codereview.chromium.org/230503002

git-svn-id: svn://svn.chromium.org/blink/trunk@171424 bbb929c8-8fbe-4397-9dbb-9b2b20218538