This depends on the possibility of creating browsing contexts with an opener directly

First try: https://treeherder.mozilla.org/#/jobs?repo=try&revision=e1e07a60da9990276a5b34dc60116a8d61cb241a&group_state=expanded&selectedJob=213144916

By replacing nsWebBrowser's implementation of the nsIBaseWindow.initWindow and nsIBaseWindow.create with a new static nsWebBrowser::Create method we make it possible to pass arguments directly when creating an nsWebBrowser, for example the opener BrowsingContext. As a bonus we can do away with nsWebBrowser::mInitInfo!

Try looked good, and I could explain all oranges except three Android JIT which must be unrelated, so I'll go ahead and queue this for landing.

Pushed by afarre@mozilla.com: https://hg.mozilla.org/integration/autoland/rev/cb9dec83210d Create BrowsingContext with passed opener. r=qdot