Make <webview> use out-of-process iframe architecture.

chrome/test/data/extensions/platform_apps/web_view/post_message/embedder.js

+// Copyright 2015 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 = null;
+
+var LOG = function(msg) {
+  window.console.log(msg);
+};
+
+var startTest = function(guestURL) {
+  window.onmessage = onMessge;
+  chrome.test.sendMessage('guest-loaded');
+  webview = document.getElementById('webview');
+  webview.onloadstop = onWebviewLoaded;
+  webview.onconsolemessage = function(e) { LOG('[Guest]: ' + e.message); };
+
+  webview.setAttribute('src', guestURL);
+};
+
+var onWebviewLoaded = function(event) {
+  window.console.log('onWebviewLoaded');
+  webview.contentWindow.postMessage(JSON.stringify(['ping']), '*');
+};
+
+var onMessge = function(e) {
+  var data = JSON.parse(e.data);
+  if (data.length != 1 && data[0] !== 'pong') {
+    chrome.test.fail();
+  } else {
+    chrome.test.succeed();
+  }
+};
+
+chrome.test.getConfig(function(config) {
+  chrome.test.runTests([
+      function postMessage() {
+        var guestURL = 'http://localhost:' + config.testServer.port +
+            '/extensions/platform_apps/web_view/post_message/guest.html';
+        LOG('guestURL: ' + guestURL);
+        document.querySelector('#webview-tag-container').innerHTML =
+            '<webview id="webview"></webview>';
+        startTest(guestURL);
+      }]);
+});