### Preflight Checklist
* [x] I have read the [Contributing Guidelines](https://github.com/electron/electron/blob/master/CONTRIBUTING.md) for this project.
* [x] I agree to follow the [Code of Conduct](https://github.com/electron/electron/blob/master/CODE_OF_CONDUCT.md) that this project adheres to.
* [x] I have searched the issue tracker for an issue that matches the one I want to file, without success.

### Issue Details

* **Electron Version:**
10.0.0-nightly.20200220
* **Operating System:**
macOS 10.15.1
* **Last Known Working Electron version:**
I don't know (judging by implementation it never worked)

### Expected Behavior:
Call to BrowserWindow.webContents.focus focuses proper window

### Actual Behavior:
Call  to BrowserWindow.webContents.focus does nothing

### To Reproduce:

Run following simple app:
```
const { app, BrowserWindow } = require('electron')

function createWindow () {
  let win1 = new BrowserWindow({ x:100, y: 300, width: 300, height: 200 })
  win1.on('closed', () => {
    win1 = null
  })
  win1.loadURL('https://github.com')

  let win2 = new BrowserWindow({ x:300, y: 300, width: 300, height: 200 })
  win2.on('closed', () => {
    win1 = null
  })
  win2.loadURL('http://www.example.com')

  setTimeout(() => {
    win1.webContents.focus();
  }, 3000);
}

app.on('ready', createWindow)
```

It works as expected on Windows: it creates two windows and after three seconds it focuses the first one. On mac it doesn't work and call to `webContents.focus` doesn't change the focus (it stays on the second window).
shell/browser/api/electron_api_web_contents.cc
// LoadURLWithParams() triggers JS events which can call destroy() on |this|.
  // It's not safe to assume that |this| points to valid memory at this point.
  if (!weak_this)
    return;
  // Set the background color of RenderWidgetHostView.
  // We have to call it right after LoadURL because the RenderViewHost is only
  // created after loading a page.
  auto* const view = weak_this->ontents()->GetRenderWidgetHostView();
  if (view) {
    auto* web_preferences = WsPreferences::From(ontents());
    std::string color_name;
    if (web_preferences->GetPreference(options::kBackgroundColor,
                                       &color_name)) {
      view->SetBackgroundColor(ParseHexColor(color_name));
    } else {
      view->SetBackgroundColor(SK_ColorTRANSPARENT);
    }
  }
}
void Ws::DownloadURL(const GURL& url) {
  auto* browser_context = ontents()->GetBrowserContext();
  auto* download_manager =
      content::BrowserContext::GetDownloadManager(browser_context);
  std::unique_ptr<download::DownloadUrlParameters> download_params(
      content::DownloadRequestUtils::CreateDownloadForWsMainFrame(
          ontents(), url, MISSING_TRAFFIC_ANNOTATION));
  download_manager->DownloadUrl(std::move(download_params));
}
GURL Ws::GetURL() const {
  return ontents()->GetURL();