Web Animations: Avoid redundant timing updates.

This avoids a second style recalc when pending CSS Animations are started
immediately.

Review URL: https://codereview.chromium.org/86633002

git-svn-id: svn://svn.chromium.org/blink/trunk@162664 bbb929c8-8fbe-4397-9dbb-9b2b20218538