Web Animations: Schedule restart of compositor animations following a timing change

When a change to the timing of an animation was made the system cancels
any corresponding animations and falls back to animation on the main
thread.

r171123[1] added support for starting compositor animations with fixed
start times. That allows this patch to schedule animations to attempt
a restart on the compositor following a timing change. The new compositor
animation is given a start time that seeks to the current position in
the animation.

[1] http://src.chromium.org/viewvc/blink?view=rev&rev=171123

BUG=360471,352422

Review URL: https://codereview.chromium.org/230173003

git-svn-id: svn://svn.chromium.org/blink/trunk@171741 bbb929c8-8fbe-4397-9dbb-9b2b20218538