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
void DocumentTimeline::dispatchEvents()
{
    Vector<EventToDispatch> events = m_events;
    m_events.clear();
    for (size_t i = 0; i < events.size(); i++)
        events[i].target->dispatchEvent(events[i].event.release());
}

void DocumentTimeline::dispatchEvents()
{
    Vector<EventToDispatch> events = m_events;
    m_events.clear();
    for (size_t i = 0; i < events.size(); i++)
        events[i].target->dispatchEvent(events[i].event.release());
}
