MD Settings: On Startup, extract dialog to its own file.

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

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+/**
+ * @fileoverview 'settings-startup-url-dialog' is a component for adding
+ * or editing a startup URL entry.
+ */
+Polymer({
+  is: 'settings-startup-url-dialog',
+
+  properties: {
+    /** @private {string} */
+    url_: String,
+  },
+
+  /** @private {!settings.SearchEnginesBrowserProxy} */
+  browserProxy_: null,
+
+  /** @override */
+  ready: function() {
+    this.browserProxy_ = settings.StartupUrlsPageBrowserProxyImpl.getInstance();
+    this.$.add.disabled = true;
+  },
+
+  /** @override */
+  attached: function() {
+    this.$.dialog.open();
+  },
+
+  /** @private */
+  onCancelTap_: function() {
+    this.$.dialog.close();
+  },
+
+  /** @private */
+  onAddTap_: function() {
+    this.browserProxy_.addStartupPage(this.url_);
+    this.$.dialog.close();
+  },
+
+  /** @private */
+  validate_: function() {
+    this.browserProxy_.validateStartupPage(this.url_).then(function(isValid) {
+      this.$.add.disabled = !isValid;
+    }.bind(this));
+  },
+});