[embedlite-components] Ensure restored history entries have a trigger…

…ingPrincipal. Fixes JB#51572

When the history entries are restored when the browser starts up, or on
a tab change, the triggeringPrincipal must be set on them, otherwise
DocShell won't allow them to be used with the GoBack()/GoForwards()
methods.

This change adds a NullPrincipal(), which is considered a valid value
(i.e. not a null pointer).

jsscripts/embedhelper.js

@@ -331,6 +331,7 @@ EmbedHelper.prototype = {
             }
             let historyEntry = Cc["@mozilla.org/browser/session-history-entry;1"].createInstance(Ci.nsISHEntry);
             historyEntry.setURI(uri);
+            historyEntry.triggeringPrincipal = Services.scriptSecurityManager.createNullPrincipal({});
             shist.addEntry(historyEntry, true);
         });
         if (index < 0) {