* 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
if (!Browser::Get()->is_ready()) {
    args->ThrowError("Cannot create BrowserWindow before app is ready");
    return nullptr;
  }
  if (args->Length() > 1) {
    args->ThrowError();
    return nullptr;
  }
  mate::Dictionary options;
  if (!(args->Length() == 1 && args->GetNext(&options))) {
    options = mate::Dictionary::CreateEmpty(args->isolate());
  }
  return new BrowserWindow(args->isolate(), args->GetThis(), options);
}
void BrowserWindow::BuildPrototype(v8::Isolate* isolate,
                                   v8::Local<v8::FunctionTemplate> prototype) {
  prototype->SetClassName(mate::StringToV8(isolate, "BrowserWindow"));
  mate::ObjectTemplateBuilder(isolate, prototype->PrototypeTemplate())
      .SetMethod("focusOnWebView", &BrowserWindow::FocusOnWebView)
      .SetMethod("blurWebView", &BrowserWindow::BlurWebView)
      .SetMethod("isWebViewFocused", &BrowserWindow::IsWebViewFocused)
      .SetProperty("webContents", &BrowserWindow::GetWebContents);
}
v8::Local<v8::Value> BrowserWindow::From(v8::Isolate* isolate,