This actor can be used for communicating with individual frames, without
depending on walking the tree in the content process.
This is not yet complete. No tests have been written for it, the
WindowGlobalParent objects need to be exposed to chrome JS, and a form of JS
actors should be installed under them.
In addition, BrowsingContextChrome objects should be updated to allow access to
the current WindowGlobalParent in that context.
Differential Revision: https://phabricator.services.mozilla.com/D4623
This will be useful as a basis for asynchronous actors which would like to exist
both when crossing the process boundary (managed by PContent), and when
displaying an in-process window.
Differential Revision: https://phabricator.services.mozilla.com/D4622