Fix typo in js code when checking the origin of a message coming from the

chrome/browser/resources/gaia_auth_host/authenticator.js

Index: chrome/browser/resources/gaia_auth_host/authenticator.js
diff --git a/chrome/browser/resources/gaia_auth_host/authenticator.js b/chrome/browser/resources/gaia_auth_host/authenticator.js
index 59279fe3fcd21512816419db01ba463ec2d42130..3623a43fe5dff853fd0ac898e176755fd74c893b 100644
--- a/chrome/browser/resources/gaia_auth_host/authenticator.js
+++ b/chrome/browser/resources/gaia_auth_host/authenticator.js
@@ -248,7 +248,7 @@ cr.define('cr.login', function() {
    */
   Authenticator.prototype.onMessageFromWebview_ = function(e) {
     // The event origin does not have a trailing slash.
-    if (e.origin != this.idpOrigin_.substring(0, this.idpOrigin_ - 1)) {
+    if (e.origin != this.idpOrigin_.substring(0, this.idpOrigin_.length - 1)) {
       return;
     }