Allow toString on InstallTrigger (toSource is already allowed, bug 628410).

toolkit/mozapps/extensions/content/extensions-content.js

@@ -62,6 +62,7 @@ function createInstallTrigger(window) {
       install: "r",
       installChrome: "r",
       startSoftwareUpdate: "r",
+      toString: "r",
       toSource: "r", // XXX workaround for bug 582100
     },