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
size_t DocumentTimeline::numberOfActiveAnimationsForTesting() const
{
    if (isNull(m_zeroTime))
        return 0;
    if (isNull(m_zeroTime))
        return 0;
    size_t count = 0;
    for (size_t i = 0; i < m_players.size(); ++i) {
        const TimedItem* timedItem = m_players[i]->source();
        if (m_players[i]->hasStartTime())
            count += (timedItem && (timedItem->isCurrent() || timedItem->isInEffect()));
    }
    return count;
}

size_t DocumentTimeline::numberOfActiveAnimationsForTesting() const
{
    if (isNull(m_zeroTime))
        return 0;
    if (isNull(m_zeroTime))
        return 0;
    size_t count = 0;
    for (size_t i = 0; i < m_players.size(); ++i) {
        const TimedItem* timedItem = m_players[i]->source();
        if (m_players[i]->hasStartTime())
            count += (timedItem && (timedItem->isCurrent() || timedItem->isInEffect()));
    }
    return count;
}
