web-extension: Stop using unstable DOM API headers

It's undesirable for Yelp to live on the edge, depending on API/ABI that
can change at any time, without us noticing, without any WebKit soname
bump. So the unstable API has been removed from WebKit. All API needed
by Yelp is now stable.

And there was much rejoicing in the streets!

https://bugzilla.gnome.org/show_bug.cgi?id=773551