Web Animations: Temporary fix for performance regression in transition tests

This patch adds a flag to Player::setStartTime to permit it to only call
update() instead of serviceAnimations() (as per previous behaviour). We
intend to in the near future replace the serviceAnimations calls with a
deferred timing update system. Note that update() here is not correct in
general as we may need to ask the timeline to request frame callbacks.

BUG=340511

Review URL: https://codereview.chromium.org/155303002

git-svn-id: svn://svn.chromium.org/blink/trunk@166473 bbb929c8-8fbe-4397-9dbb-9b2b20218538