After the changes in this bug, about:blank loads triggered by chrome will finish in a "web" content process, as they have an untrusted null principal without a precursor. In a few places throughout the codebase, however, we perform about:blank loads with the explicit expectation that they do not change processes. This new remoteTypeOverride option allows the intended final process to be explicitly specified in this situation. For security & simplicity reasons, this new attribute is limited to only be usable on system-principal triggered loads of about:blank in toplevel browsing contexts. Differential Revision: https://phabricator.services.mozilla.com/D120671