Identity API: Change WebAuthFlow to use <webview> instead of a normal browser pop-up.

chrome/browser/resources/identity_api_ui/scope_approval_dialog.js

+// Copyright 2013 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.
+
+var webview = document.querySelector('webview');
+
+function start(url, win) {
+  webview.src = url;
+  if (win) {
+    webview.addEventListener('loadstop', function() {
+      win.show();
+    });
+  }
+}
+
+document.querySelector('.titlebar-close-button').onclick = function() {
+  window.close();
+};
+
+chrome.identityPrivate.getResources(function(resources) {
+  var style = document.styleSheets[0];
+
+  function insertRule(selector, url) {
+    style.insertRule(selector + ' { background-image: url(' + url + '); }',
+                     style.cssRules.length);
+  }
+
+  insertRule('.titlebar-close-button', resources.IDR_CLOSE_DIALOG);
+  insertRule('.titlebar-close-button:hover', resources.IDR_CLOSE_DIALOG_H);
+  insertRule('.titlebar-close-button:active', resources.IDR_CLOSE_DIALOG_P);
+});