Bug 1826645 Part 2: Make Document hold fullscreen requests while an exit is...

Bug 1826645 Part 2: Make Document hold fullscreen requests while an exit is being processed. r=edgar

This change makes the parent process delay a fullscreen request if there
is a pending fullscreen exit. It also changes the DOMFullscreenParent
actor listener lifecycle. Once it has started handling a fullscreen
request, it will remain a listener to the Document until it receives an
exit event when the manager is out of fullscreen.

Differential Revision: https://phabricator.services.mozilla.com/D175186