sandboxed top-level navigation browsing context flag

sandboxed top-level navigation browsing context flag

[1] Add allow-top-navigation-by-user-activation sandbox token (domenic著, ) https://github.com/whatwg/html/commit/8abd73ec3a56541e3f6486d3a593a8bcfd67295d

[2] New in Chrome 58  |  Web  |  Google Developers () https://developers.google.com/web/updates/2017/04/nic58

[3] Revamp the user activation model (mustaqahmed, , ) https://github.com/whatwg/html/commit/8f8c1f50158736b3cf16188377a0974a20367c8b