Web Animations CSS: Avoid waiting for cancelled compositor animation

When a renderer associated with a composited animation is destroyed, other
animations waiting for a synchronized start may never be notified. We can
detect this case and start such animations immediately. After this change
CSSPendingAnimations now requests service on every frame until all pending
animations have a chance to start.

BUG=322272

Review URL: https://codereview.chromium.org/83793017

git-svn-id: svn://svn.chromium.org/blink/trunk@162678 bbb929c8-8fbe-4397-9dbb-9b2b20218538