Bug 1478208 - Implement HTMLMediaElement.allowedToPlay. r=alwu,bz

Various web authors have expressed desire to know in advance whether autoplay
will work.

They want this in order to avoid paying the price for downloading media that
won't play. Or they want to take other action such as showing a poster image
instead.

This is of particular interest to Firefox, as we're planning on showing a
prompt to ask the user whether they would like a site to play. If sites want to
determine whether they can autoplay but avoid the prompt showing, they won't be
able to just call play() in Firefox and see whether it works, as that would
likely show the prompt if the user doesn't already have a stored permission.

We've been working out a spec here:
https://github.com/whatwg/html/issues/3617#issuecomment-398613484

This implements what is the consensus to date there;
HTMLMediaElement.allowedToPlay, which returns true when a play() call would not
be blocked with NotAllowedError by autoplay blocking policies.

MozReview-Commit-ID: AkBu0G7uCJ0

dom/html/HTMLMediaElement.cpp

@@ -2522,6 +2522,12 @@ HTMLMediaElement::ResumeLoad(PreloadAction aAction)
   }
 }
 
+bool
+HTMLMediaElement::AllowedToPlay() const
+{
+  return AutoplayPolicy::IsAllowedToPlay(*this) == nsIAutoplay::ALLOWED;
+}
+
 void
 HTMLMediaElement::UpdatePreloadAction()
 {