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* 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

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atom/browser/web_contents_preferences.cc
content::WebPreferences* prefs) {
  prefs->javascript_enabled = IsEnabled("javascript", true );
  prefs->images_enabled = IsEnabled("images", true );
  prefs->text_areas_are_resizable =
      IsEnabled("textAreasAreResizable", true );
  prefs->navigate_on_drag_drop =
      IsEnabled("navigateOnDragDrop", false );
  
  bool is_webgl_enabled = IsEnabled("webgl", true );
  prefs->webgl1_enabled = is_webgl_enabled;
  prefs->webgl2_enabled = is_webgl_enabled;
  
  bool is_web_security_enabled =
      IsEnabled(options::kWebSecurity, true );
  prefs->web_security_enabled = is_web_security_enabled;
  prefs->allow_running_insecure_content =
      IsEnabled(options::kAllowRunningInsecureContent,
                !is_web_security_enabled );
  auto* fonts_dict = preference_.FindKeyOfType("defaultFontFamily",