* 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
mate::Dictionary web_preferences = mate::Dictionary::CreateEmpty(isolate);
  options.Get(options::kWebPreferences, &web_preferences);
  
  v8::Local<v8::Value> value;
  if (options.Get(options::kBackgroundColor, &value))
    web_preferences.Set(options::kBackgroundColor, value);
  v8::Local<v8::Value> transparent;
  if (options.Get("transparent", &transparent))
    web_preferences.Set("transparent", transparent);
  if (options.Get("webContents", &web_contents) && !web_contents.IsEmpty()) {
    
    
    
    auto* existing_preferences =
        WebContentsPreferences::From(web_contents->web_contents());