Devtools Animations: Listen to all animation play state changes

This change adds animation instrumentation for all play state changes. The animation agent then decides how to use this information to ensure that the front-end is always notified of start times where applicable and where start times will not be received. This ensures our pending animations are always flushed in a timely fashion.

BUG=447083,554266

Review URL: https://codereview.chromium.org/1422713012

Cr-Commit-Position: refs/heads/master@{#359383}
12 files changed