[cssom] Sprinkle "[CEReactions]" IDL extended attributes

[zcorpan]

From whatwg/html#1412 (comment)

Anything that can cause the style content attribute to change for an HTML element needs to have a [CEReactions] IDL extended attribute, for custom elements.

https://drafts.csswg.org/cssom/#cssstyledeclaration

e.g. x.style.cssFloat = 'left'
http://software.hixie.ch/utilities/js/live-dom-viewer/saved/4272