* Electron version: 2.0.0-beta.7
* Operating system: macOS 10.13.4

### Expected behavior

<!-- What do you think should happen? -->
When HTML fullscreen mode is active, exiting fullscreen on the window level (clicking the green button or calling `win.setFullScreen(false)` should also exit HTML fullscreen mode.

### Actual behavior

HTML fullscreen mode stays active. Chrome correctly exits HTML fullscreen mode.

### How to reproduce

Use console in the default_app:

```
document.addEventListener('webkitfullscreenchange', () => console.log('webkitfullscreenchange'));
document.documentElement.webkitRequestFullscreen();
```

now exit fullscreen by clicking the green button or run `require('electron').remote.getCurrentWindow().setFullScreen(false);`

Compare to what happens when you do the same in Chrome/Chromium
`webkitfullscreenchange` should be logged twice (once for enter, once for exit)
`document.webkitFullscreenElement` should also be `null`

Possible workaround is:
```
win.on('leave-full-screen', () => {
  win.webContents.executeJavaScript('document.webkitExitFullscreen();');
});
```

atom/browser/api/atom_api_browser_window.cc
base::DictionaryValue web_preferences_dict;
    if (mate::ConvertFromV8(isolate, web_preferences.GetHandle(),
                            &web_preferences_dict)) {
      existing_preferences->dict()->Clear();
      existing_preferences->Merge(web_preferences_dict);
    }
  } else {
    
    web_contents = WebContents::Create(isolate, web_preferences);
  }
  web_contents_.Reset(isolate, web_contents.ToV8());
  api_web_contents_ = web_contents.get();
  api_web_contents_->AddObserver(this);
  Observe(api_web_contents_->web_contents());
  
  mate::Dictionary(isolate, web_contents->GetWrapper())
      .Set("browserWindowOptions", options);
  
  
  ui::GpuSwitchingManager::SetTransparent(window()->transparent());
  
  web_contents->SetOwnerWindow(window());
  auto* host = web_contents->web_contents()->GetRenderViewHost();
  if (host)
    host->GetWidget()->AddInputEventObserver(this);
  InitWith(isolate, wrapper);
  
  window()->InitFromOptions(options);
}
BrowserWindow::~BrowserWindow() {