Web Animations: Avoid outdating players when setting start times

This avoids spurious on-demand updates.

BUG=368841

Review URL: https://codereview.chromium.org/273523003

git-svn-id: svn://svn.chromium.org/blink/trunk@173493 bbb929c8-8fbe-4397-9dbb-9b2b20218538