Intent to prototype ARIA reflection (non-IDREF)

[Alexander]

/Summary/: ARIA reflection is an extension of Element interface, which
provides a handy way to manage ARIA attributes on a DOM element, what
makes web authors life easier. Note, IDREF attributes part is not
included at this point (see https://github.com/whatwg/html/issues/4925).

/Bug/: https://bugzilla.mozilla.org/show_bug.cgi?id=1628418

/Standard/: https://w3c.github.io/aria/#idl-interface

/Platform coverage/: web

/Preference/: accessibility.ARIAReflection.enable

/DevTools bug/: no extra effort should be required since the interfaces
are extensions of Element interface and should be picked up
automatically by DevTools.

/Other browsers/:

* Safari shipped (/Aug 2018, https://bugs.webkit.org/show_bug.cgi?id=184676)
/

* Chrome shipped behind a flag (Jun 2018,
https://bugs.chromium.org/p/chromium/issues/detail?id=844540)

/web-platform-tests/:
https://searchfox.org/mozilla-central/source/testing/web-platform/tests/dom/nodes/aria-attribute-reflection.tentative.html

/Secure contexts/: security/privacy is scoped by the attribute
reflection
https://html.spec.whatwg.org/multipage/common-dom-interfaces.html#reflecting-content-attributes-in-idl-attribute,
which shouldn't have any security/privacy issues, since API doesn't
allows anything beyond setAttribute/getAttribute.

/Is this feature enabled by default in sandboxed iframes?/ yes

[James Teh]

I intend to enable this in Nightly to gather feedback in the wild and make further progress towards getting this shipped. There is one remaining spec issue that still needs to be resolved, but there seems to be good progress there now.
Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1824980

Jamie