Bug 1650089 - Part 1: Add a remoteTypeOverride option for about:blank loads triggered by chrome, r=annyG,kmag

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

browser/components/sessionstore/SessionStore.jsm

@@ -3649,6 +3649,7 @@ var SessionStoreInternal = {
       triggeringPrincipal: Services.scriptSecurityManager.createNullPrincipal({
         userContextId: aTab.userContextId,
       }),
+      remoteTypeOverride: E10SUtils.NOT_REMOTE,
     });
 
     let data = TabState.collect(aTab, TAB_CUSTOM_VALUES.get(aTab));