Implement frameElement for Window

This does not perform proper cross-origin security checks, since that is not possible in jsdom's current setup.

Author: morchard

lib/jsdom/browser/Window.js

@@ -61,6 +61,7 @@ function Window(options) {
   // Set up the window as if it's a top level window.
   // If it's not, then references will be corrected by frame/iframe code.
   this._parent = this._top = this._globalProxy;
+  this._frameElement = null;
 
   // List options explicitly to be clear which are passed through
   this._document = Document.create([], {
@@ -122,6 +123,9 @@ function Window(options) {
     get window() {
       return window._globalProxy;
     },
+    get frameElement() {
+      return window._frameElement;
+    },
     get frames() {
       return window._globalProxy;
     },

lib/jsdom/living/nodes/HTMLFrameElement-impl.js

@@ -58,6 +58,7 @@ function loadFrame(frame) {
   const contentWindow = contentDoc._defaultView;
   contentWindow._parent = parent;
   contentWindow._top = parent.top;
+  contentWindow._frameElement = frame;
   contentWindow._virtualConsole = parent._virtualConsole;
 
   // Handle about:blank with a simulated load of an empty document.