Web Animations API: Start compositor animations for element.animate and document.timeline.play

Modifies CSSPendingAnimations (now renamed to CompositorPendingAnimations)
to handle the starting of animations with a specified start time. Animations
without a start time are considered to be 'synchronized' and wait for
notification of a start time from the compositor.

Depends on: https://src.chromium.org/viewvc/chrome?view=rev&revision=262582

BUG=360471

Review URL: https://codereview.chromium.org/225453002

git-svn-id: svn://svn.chromium.org/blink/trunk@171123 bbb929c8-8fbe-4397-9dbb-9b2b20218538