Intent to ship: ShadowRoot's delegatesFocus attribute

[Sean Feng]

Summary: delegatesFocus is a new attribute for ShadowRoot which introduces some new focusing behaviours when the host is focused.

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1413836

Standard: https://dom.spec.whatwg.org/#dom-shadowroot-delegatesfocus

Platform coverage: all

Preference: dom.shadowdom.delegatesFocus.enabled

Other browsers: Both Chrome and Safari support the feature

Web Platform Tests: https://github.com/web-platform-tests/wpt/tree/master/shadow-dom/focus

Thanks,

Sean