ARIA IDL miss CEReaction attribute

[asurkov]

@smaug---- pointed out [1] that AccessibilityRole and ARIAAttributes interfaces [2] miss CEReactions webidl attribute, because they all are about changing corresponding DOM attributes and thus Custom Elements if any should be handle that properly.

[1] https://phabricator.services.mozilla.com/D71169#inline-418480
[2] https://rawgit.com/w3c/aria/master/#AccessibilityRole

[smaug----]

https://html.spec.whatwg.org/#cereactions

[cookiecrook]

We should probably address this along with the other potential IDL changes in #1110

[cookiecrook]

WebKit tracker: https://webkit.org/b/231049
Both Chrome and Firefox already implement.

[rniwa]

WPT: web-platform-tests/wpt#34870