Add support for native chromium popups on sandboxed renderers.

- Allow `api::Window` instances to be created from existing `api::WebContents`.
- Override `WebContentsCreated` and `AddNewContents` to wrap renderer-created
  `content::WebContents` into `api::WebContents`.
- For `content::WebContents` that should be displayed in new windows, pass the
  wrapped `api::WebContents` object to window manager.

atom/browser/native_window.cc

@@ -391,6 +391,10 @@ void NativeWindow::RequestToClosePage() {
   if (window_unresposive_closure_.IsCancelled())
     ScheduleUnresponsiveEvent(5000);
 
+  if (!web_contents())
+    // Already closed by renderer
+    return;
+
   if (web_contents()->NeedToFireBeforeUnload())
     web_contents()->DispatchBeforeUnload();
   else