Web Animations: Pull timeToEffectChange out of Player::update

There's really no reason for timeToEffectChange being a return argument
to Player::update instead of a separate function. Pulling it out makes
the logic a bit simpler.

NOTRY=true
BUG=334926

Review URL: https://codereview.chromium.org/169253002

git-svn-id: svn://svn.chromium.org/blink/trunk@167299 bbb929c8-8fbe-4397-9dbb-9b2b20218538