Notification actions may have an icon url.

This is behind a runtime flag for two reasons:
* The implementation is incomplete.
* We're still evaluating the API design.

Intent to Implement and Ship: Notification Action Icons
https://groups.google.com/a/chromium.org/d/msg/blink-dev/IM0HxOP7HOA/y8tu6iq1CgAJ

BUG=581336

Review URL: https://codereview.chromium.org/1644573002

Cr-Commit-Position: refs/heads/master@{#374649}
void Notification::dispatchCloseEvent()
{
    if (m_state != NotificationStateShowing && m_state != NotificationStateClosing)
        return;

    m_state = NotificationStateClosed;
    dispatchEvent(Event::create(EventTypeNames::close));
}

void Notification::dispatchCloseEvent()
{
    if (m_state != NotificationStateShowing && m_state != NotificationStateClosing)
        return;

    m_state = NotificationStateClosed;
    dispatchEvent(Event::create(EventTypeNames::close));
}
