Bug 1650089 - Part 6: Don't set maychangeremoteness on non-e10s browsers, r=Gijs

This change makes all browsers which were not created with an initial `remote`
attribute within a non-`useRemoteTabs` window be unable to process-switch, as
otherwise we may attempt to switch loads into a content process. We need to
keep process switching enabled for explicitly-remote browsers loaded in a
non-`useRemoteTabs` window as it's relied on for tests and can lead to
assertion failures due to loading remote content in the parent process.

Differential Revision: https://phabricator.services.mozilla.com/D121285

browser/base/content/tabbrowser.js

@@ -2095,7 +2095,6 @@
 
       const defaultBrowserAttributes = {
         contextmenu: "contentAreaContextMenu",
-        maychangeremoteness: "true",
         message: "true",
         messagemanagergroup: "browsers",
         selectmenulist: "ContentSelectDropdown",
@@ -2106,6 +2105,10 @@
         b.setAttribute(attribute, defaultBrowserAttributes[attribute]);
       }
 
+      if (gMultiProcessBrowser || remoteType) {
+        b.setAttribute("maychangeremoteness", "true");
+      }
+
       if (!initiallyActive) {
         b.setAttribute("initiallyactive", "false");
       }