Extract the element implementation logic to function mods in <webview>.

extensions/renderer/resources/guest_view/guest_view_iframe_container.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.
+
+// --site-per-process overrides for guest_view_container.js
+
+var GuestViewContainer = require('guestViewContainer').GuestViewContainer;
+
+GuestViewContainer.prototype.createInternalElement$ = function() {
+  var iframeElement = document.createElement('iframe');
+  iframeElement.style.width = '100%';
+  iframeElement.style.height = '100%';
+  privates(iframeElement).internal = this;
+  return iframeElement;
+};