Currently, TabGroups know to break their reference cycles only when the last
window leaves them. For TabGroups which have never had a window join (which
happens under Fission), this means they also never see a window leave, and
therefore never break their reference cycles, and leak.
This patch adds a check to break reference cycles if no windows have joined by
the time a BrowserChild they belong to is destroyed.
MANUAL PUSH: Lando fails to rebase.
Differential Revision: https://phabricator.services.mozilla.com/D40669