[w3c/webcomponents] fullscreenchange should be dispatched on each ancestor shadow root (#614)

As things stand, [requesting a fullscreen](https://fullscreen.spec.whatwg.org/#dom-element-requestfullscreen) only fires `fullscreenchange` event on each "ancestor” documents.

This would mean that any component that contains an element that can fullscreen, it needs to listen to  `fullscreenchange` event on a document. That’s pretty clumsy.

I can think of two options here:
 1. Make `fullscreenchange` both composed and bubble, and dispatch it from the fullscreen’ed element and their ancestor document instead of just on ancestor documents.  Of course, if an iframe appears within another shadow tree, we would fire an event at the iframe and bubble up, composed, as well.
 2. In addition to ancestor documents, dispatch the event on every ancestor shadow root.

I mildy prefer (1) just because WebKit’s `webkitfullscreenchange` event is fired on the fullscreen’ed element as is.  However, (1) wouldn’t work for `fullscreenerror` event.  This is probably why `fullscreenchange` is spec’ed the way it is now.

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