Web Animations: Don't eagerly assign start times for element.animate

This ensure that animations started by element.animate function the same way
as regular CSS Animations -- they will wait for notification of a start
time from the compositor.

BUG=389396

Review URL: https://codereview.chromium.org/354123002

git-svn-id: svn://svn.chromium.org/blink/trunk@177071 bbb929c8-8fbe-4397-9dbb-9b2b20218538