Bug 1911745 - Unify BrowsingContext flag coherency checks, r=mccr8

Previously these checks were largely diagnostic tools for finding bugs
in other code as it evolves. This unifies the checks a bit more and
makes them stronger for BrowsingContexts created over IPC, providing a
place for more coherency checks to be added in the future.

Differential Revision: https://phabricator.services.mozilla.com/D218860