Bug 593026. Make it possible to get the system principal from script. r=jst a=jst

caps/idl/nsIScriptSecurityManager.idl

@@ -184,7 +184,7 @@ interface nsIScriptSecurityManager : nsIXPCSecurityManager
     /**
      * Return the all-powerful system principal.
      */
-    [noscript] nsIPrincipal getSystemPrincipal();
+    nsIPrincipal getSystemPrincipal();
 
     /**
      * Return a principal with the specified certificate fingerprint, subject