* 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
content::RenderViewHost* render_view_host) {
  if (!window()->transparent())
    return;
  content::RenderWidgetHostImpl* impl = content::RenderWidgetHostImpl::FromID(
      render_view_host->GetProcess()->GetID(),
      render_view_host->GetRoutingID());
  if (impl)
    impl->SetBackgroundOpaque(false);
}
void BrowserWindow::DidFirstVisuallyNonEmptyPaint() {
  if (window()->IsVisible())
    return;
  
  
  auto* const view = web_contents()->GetRenderWidgetHostView();
  view->Show();
  view->SetSize(window()->GetContentSize());
  
  base::ThreadTaskRunnerHandle::Get()->PostTask(
      FROM_HERE, base::BindOnce(
                     [](base::WeakPtr<BrowserWindow> self) {
                       if (self)
                         self->Emit("ready-to-show");
                     },
                     GetWeakPtr()));
}
void BrowserWindow::BeforeUnloadDialogCancelled() {