Issue #1332 - Backport promise-based media playback

https://bugzilla.mozilla.org/show_bug.cgi?id=1244768

I happened to find an older version of the promise-based media playback patch in Bugzilla, the one that was originally submitted for review. It had the DocShell changes I already knew how to deal with, and had fewer of the audio wrapper and nsISupports changes that were confusing me in the later patch. I was able to do a better job getting this back into a UXP-appropriate configuration than I could have with the final version.

I'm honestly still a little unsure about some of the minor refactoring done in the patch itself, insisting on already_AddRefed promises and such, but I don't really know how to avoid those completely. Still, I think it's better than it was.

dom/webidl/HTMLMediaElement.webidl

@@ -68,7 +68,7 @@ interface HTMLMediaElement : HTMLElement {
   [SetterThrows]
            attribute boolean loop;
   [Throws]
-  void play();
+  Promise<void> play();
   [Throws]
   void pause();