MD History: fix script errors when navigating to /syncedTabs (or refreshing)

chrome/browser/resources/md_history/app.js

Index: chrome/browser/resources/md_history/app.js
diff --git a/chrome/browser/resources/md_history/app.js b/chrome/browser/resources/md_history/app.js
index 7cc406ef400055dc7b2de60a0eed8bbc1c8ca686..e0b6ffa3eaf35b3437167a68143ae58f045050fd 100644
--- a/chrome/browser/resources/md_history/app.js
+++ b/chrome/browser/resources/md_history/app.js
@@ -117,12 +117,6 @@ Polymer({
     document.addEventListener('command', this.onCommand_.bind(this));
   },
 
-  /** @override */
-  attached: function() {
-    this.addWebUIListener('sign-in-state-updated',
-                          this.updateSignInState.bind(this));
-  },
-
   onFirstRender: function() {
     setTimeout(function() {
       chrome.send(