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

extensions/renderer/resources/guest_view/web_view/web_view_iframe.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.
+
+// This module provides --site-per-process overrides for WebView (<webview>).
+// See web_view.js for details.
+
+var IdGenerator = requireNative('id_generator');
+var WebViewImpl = require('webView').WebViewImpl;
+// NOTE: Do not remove these, we implicitly depend on these in
+// --site-per-process.
+var GuestViewIframe = require('guestViewIframe');
+var GuestViewIframeContainer = require('guestViewIframeContainer');
+
+WebViewImpl.prototype.attachWindow$ = function(opt_guestInstanceId) {
+  // If |opt_guestInstanceId| was provided, then a different existing guest is
+  // being attached to this webview, and the current one will get destroyed.
+  if (opt_guestInstanceId) {
+    if (this.guest.getId() == opt_guestInstanceId) {
+      return true;
+    }
+    this.guest.destroy();
+    this.guest = new GuestView('webview', opt_guestInstanceId);
+  }
+
+  var generatedId = IdGenerator.GetNextId();
+  // Generate an instance id for the container.
+  this.onInternalInstanceId(generatedId);
+  return true;
+};