We can remove isOnlyToplevelInTabGroup entirely since we have
BrowsingContext/BrowsingContextGroup exposed through
chrome-webidl. Checking if a browsing context is the only top level
(auxilliary or otherwise) is only a matter of checking that there
isn't a parent, and that the size of the browsing context group is 1.
Differential Revision: https://phabricator.services.mozilla.com/D46590
--HG--
extra : moz-landing-system : lando