<webview>: Add test to confirm the TAB key will escape the view.

chrome/test/data/extensions/platform_apps/web_view/focus/inject_focus_take_focus.js

+// Copyright 2017 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 embedder = null;
+
+var LOG = function(msg) {
+  window.console.log(msg);
+};
+
+var sendMessage = function(msgArray) {
+  if (embedder) {
+    embedder.postMessage(JSON.stringify(msgArray), '*');
+  }
+};
+
+var init = function() {
+  var input = document.createElement('input');
+  input.style.width = '100%';
+  input.addEventListener('focus', function(e) {
+    sendMessage(['focused']);
+  });
+
+  document.body.appendChild(input);
+
+  window.addEventListener('message', function(e) {
+    var data = JSON.parse(e.data);
+    embedder = e.source;
+    LOG('message, data: ' + data);
+    if (data[0] == 'connect') {
+      sendMessage(['connected']);
+    } else if (data[0] == 'request-coords') {
+      var rect = input.getBoundingClientRect();
+      sendMessage(['response-coords',
+          rect.left + 0.5 * rect.width,
+          rect.top + 0.5 * rect.height]);
+    }
+  });
+};
+
+init();