MD Settings: implement support for extension-controlled startup URLs

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

Index: chrome/browser/resources/settings/on_startup_page/startup_url_entry.js
diff --git a/chrome/browser/resources/settings/on_startup_page/startup_url_entry.js b/chrome/browser/resources/settings/on_startup_page/startup_url_entry.js
index e3e93dda2bb254b9479b25b0f104870554ddb27a..b5d363e47b21f1a153c1d241d602689d1ff14887 100644
--- a/chrome/browser/resources/settings/on_startup_page/startup_url_entry.js
+++ b/chrome/browser/resources/settings/on_startup_page/startup_url_entry.js
@@ -21,6 +21,11 @@ Polymer({
   is: 'settings-startup-url-entry',
 
   properties: {
+    editable: {
+      type: Boolean,
+      reflectToAttribute: true,
+    },
+
     /** @type {!StartupPageInfo} */
     model: Object,
   },
@@ -50,7 +55,7 @@ Polymer({
   /** @private */
   onDotsTap_: function() {
     var actionMenu = /** @type {!CrActionMenuElement} */(
-        this.$.menu.get());
-    actionMenu.showAt(assert(this.$.dots));
+        this.$$('#menu').get());
+    actionMenu.showAt(assert(this.$$('#dots')));
   },
 });