Make BrowserWindowProxy guestId non-writeable

lib/renderer/override.js

@@ -25,7 +25,13 @@ var BrowserWindowProxy = (function () {
   }
 
   function BrowserWindowProxy (guestId1) {
-    this.guestId = guestId1
+    Object.defineProperty(this, 'guestId', {
+      configurable: false,
+      enumerable: true,
+      writeable: false,
+      value: guestId1
+    })
+
     this.closed = false
     ipcRenderer.once('ELECTRON_GUEST_WINDOW_MANAGER_WINDOW_CLOSED_' + this.guestId, () => {
       BrowserWindowProxy.remove(this.guestId)