Web Animations: Don't eagerly assign start times for element.animate
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: bf5cd6ccde378db821296732a091cfbcf5285fbd@177071 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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