It seems like nsDocShell::mBrowsingContext can never be null. This is still not totally obvious to me. If this is actually the case adding nsDocShell::BrowsingContext() would make this more obvious and would short some callsite. We can also remove some null-checks.

How about using OwningNonNull or mozilla::NotNull to make the constraint obvious to the compiler?

We already have GetBrowsingContext() for this. Using the name BrowsingContext unfortunately requires every reference to the BrowsingContext type to be fully qualified, which isn't worth the trouble.