<!--
Thanks for opening an issue! A few things to keep in mind:

- The issue tracker is only for bugs and feature requests.
- Before reporting a bug, please try reproducing your issue against
  the latest version of Electron.
- If you need general advice, join our Slack: http://atom-slack.herokuapp.com
-->

* Electron version: 2.0.0-beta.7
* Operating system: Windows 10 x64

### Expected behavior

<!-- What do you think should happen? -->
When `webPreferences.nativeWindowOpen` is set to true, BrowserWindows opened by `window.open`'s memory will be released after be closed.

### Actual behavior

<!-- What actually happens? -->
The memory is not released and caused memory leak

### How to reproduce
- Clone https://github.com/KochiyaOcean/electron-bugs/tree/native-open-memory-leak
- Run `electron . ` in the cloned repo
- Memory usage of electron process will be increasing in task manager

<!-- 

Your best chance of getting this bug looked at quickly is to provide a REPOSITORY that can be cloned and run.

You can fork https://github.com/electron/electron-quick-start and include a link to the branch with your changes.

If you provide a URL, please list the commands required to clone/setup/run your repo e.g.

  $ git clone $YOUR_URL -b $BRANCH
  $ npm install
  $ npm start || electron .

-->

atom/browser/web_contents_preferences.cc
command_line->AppendSwitchASCII(::switches::kEnableBlinkFeatures, s);
  
  if (GetAsString(&preference_, options::kDisableBlinkFeatures, &s))
    command_line->AppendSwitchASCII(::switches::kDisableBlinkFeatures, s);
  if (guest_instance_id) {
    
    
    auto* manager = WebViewManager::GetWebViewManager(web_contents_);
    if (manager) {
      auto* embedder = manager->GetEmbedder(guest_instance_id);
      if (embedder) {
        auto* relay = NativeWindowRelay::FromWebContents(embedder);
        if (relay) {
          auto* window = relay->GetNativeWindow();
          if (window) {
            const bool visible = window->IsVisible() && !window->IsMinimized();
            if (!visible) {
              command_line->AppendSwitch(switches::kHiddenPage);
            }
          }
        }
      }
    }
  }
  
  
  
  last_preference_ = preference_.Clone();
}
void WebContentsPreferences::OverrideWebkitPrefs(