[MD settings] remove UI about extension controlling ntp on disable

This CL removes the note about an extension controlling the NTP from the
OnStartup URL list when the extension is disabled (since it's no longer
being controlled by that extension).

This fixes an error I'd made in CL 557971.

Bug: 740434
Change-Id: I80fea69068b24198ea28478210fbff6bebf8cb7a
Reviewed-on: https://chromium-review.googlesource.com/565998
Commit-Queue: Dave Schuyler <dschuyler@chromium.org>
Reviewed-by: Scott Chen <scottchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#485465}

chrome/browser/resources/settings/on_startup_page/startup_urls_page.js

@@ -62,10 +62,7 @@ Polymer({
 
   /** @override */
   attached: function() {
-    settings.OnStartupBrowserProxyImpl.getInstance().getNtpExtension().then(
-        function(ntpExtension) {
-          this.ntpExtension_ = ntpExtension;
-        }.bind(this));
+    this.getNtpExtension_();
 
     this.browserProxy_ = settings.StartupUrlsPageBrowserProxyImpl.getInstance();
     this.addWebUIListener('update-startup-pages', function(startupPages) {
@@ -86,6 +83,14 @@ Polymer({
     }.bind(this));
   },
 
+  /** @private */
+  getNtpExtension_: function() {
+    settings.OnStartupBrowserProxyImpl.getInstance().getNtpExtension().then(
+        function(ntpExtension) {
+          this.ntpExtension_ = ntpExtension;
+        }.bind(this));
+  },
+
   /**
    * @param {!Event} e
    * @private