Web Animations API: Ensure timing is updated if a RAF callback makes a player outdated.

Under the Web Animations API it's now possible for new animation players to be
started or existing players modified during frame callbacks. Since timing
updates are lazy it was possible for them to be delayed until the next frame.

This patch forces an update of any outdated players following the execution
of frame callbacks.

NOTRY=true

Review URL: https://codereview.chromium.org/212393002

git-svn-id: svn://svn.chromium.org/blink/trunk@170169 bbb929c8-8fbe-4397-9dbb-9b2b20218538