Correctly keep track of isolates for microtask execution

BUG=487155
R=haraken@chromium.org

Review URL: https://codereview.chromium.org/1161823002

git-svn-id: svn://svn.chromium.org/blink/trunk@195985 bbb929c8-8fbe-4397-9dbb-9b2b20218538
void Document::didChangeVisibilityState()
{
    dispatchEvent(Event::create(EventTypeNames::visibilitychange));
    dispatchEvent(Event::create(EventTypeNames::webkitvisibilitychange));

    PageVisibilityState state = pageVisibilityState();
    for (DocumentVisibilityObserver* observer : m_visibilityObservers)
        observer->didChangeVisibilityState(state);
}

void Document::didChangeVisibilityState()
{
    dispatchEvent(Event::create(EventTypeNames::visibilitychange));
    dispatchEvent(Event::create(EventTypeNames::webkitvisibilitychange));

    PageVisibilityState state = pageVisibilityState();
    for (DocumentVisibilityObserver* observer : m_visibilityObservers)
        observer->didChangeVisibilityState(state);
}
