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}
NavigatorVibration::VibrationPattern Notification::vibrate(bool& isNull) const
{
    NavigatorVibration::VibrationPattern pattern;
    pattern.appendRange(m_data.vibrate.begin(), m_data.vibrate.end());

    if (!pattern.size())
        isNull = true;

    return pattern;
}

NavigatorVibration::VibrationPattern Notification::vibrate(bool& isNull) const
{
    NavigatorVibration::VibrationPattern pattern;
    pattern.appendRange(m_data.vibrate.begin(), m_data.vibrate.end());

    if (!pattern.size())
        isNull = true;

    return pattern;
}
