Web Animations: Store the modified start time in AnimationPlayer's compositor state

The start time notification from compositor supplies timeline time, not
effective start time. Likewise, the start time supplied to the compositor
should be offset by the duration for a reversed player.

BUG=428906

Review URL: https://codereview.chromium.org/694653002

git-svn-id: svn://svn.chromium.org/blink/trunk@184705 bbb929c8-8fbe-4397-9dbb-9b2b20218538