This removes a lot of the plumbing for having the platform embed itself
through IPC which was required for B2G running the browser as both
shell and browser application.
Fullscreen stack isn't part of the spec anymore; it's been changed to a more
generic version called Top Layer stack, which is being used by both full-screen
APIs and dialog elements.
This patch refactors it to Top Layer stack so that it can be reused for dialog
elements.
Top Layer stack spec: https://fullscreen.spec.whatwg.org/#new-stacking-layer
Moonchild
Job Bautista
win7-7
roytam1