This is a fairly significant patch, however it would be difficult to break it
down into smaller patches:
1) The various mechanisms used to manage ContentParent lifecycles have been
merged together into a common "KeepAlive" system. A process will
begin shutdown when its keepalive count reaches 0. (though it will
still wait for all BrowserParents to also be dead before sending the
actual shutdown message as before).
This replaces a number of bespoke systems for tracking BrowserParent
instances in different lifecycle states, remote workers, ongoing
process switches, and preallocated processes.
2) KeepAlives are now managed automatically by a UniquePtr variant
(Unique[Threadsafe]ContentParentKeepAlive). This makes the hand-off
over KeepAlive lifecycles explicit, even for workers.
3) All KeepAlives are now keyed by a BrowserId, which will be 0 for keepalives
not associated with a specific tab. This allows the new process
selection logic to count all tabs other than the one being navigated
when deciding which process to use.
4) The process switching logic now tracks it's KeepAlive with a BrowserId,
meaning that ongoing process switches are considered when performing
process selection, even if the BrowserParent hasn't been created yet.
Differential Revision: https://phabricator.services.mozilla.com/D213338
279b50ebe8