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}
String Notification::icon() const
{
    return m_data.icon.string();
}

String Notification::icon() const
{
    return m_data.icon.string();
}
