And have it mirror in the parent process more automatically.
The docShellIsActive setter in the browser-custom-element side needs to
be there rather than in the usual DidSet() calls because the
AsyncTabSwitcher code relies on getting an exact amount of notifications
as response to that specific setter. Not pretty, but...
BrowserChild no longer sets IsActive() on the docshell itself for OOP
iframes. This fixes bug 1679521. PresShell activeness is used to
throttle rAF as well, which handles OOP iframes nicely as well.
Differential Revision: https://phabricator.services.mozilla.com/D96072
