### 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
&Ws::SetBackgroundThrottling)
      .SetMethod("getProcessId", &Ws::GetProcessID)
      .SetMethod("getOSProcessId", &Ws::GetOSProcessID)
      .SetMethod("_getOSProcessIdForFrame",
                 &Ws::GetOSProcessIdForFrame)
      .SetMethod("equal", &Ws::Equal)
      .SetMethod("_loadURL", &Ws::LoadURL)
      .SetMethod("downloadURL", &Ws::DownloadURL)
      .SetMethod("_getURL", &Ws::GetURL)
      .SetMethod("getTitle", &Ws::GetTitle)
      .SetMethod("isLoading", &Ws::IsLoading)
      .SetMethod("isLoadingMainFrame", &Ws::IsLoadingMainFrame)
      .SetMethod("isWaitingForResponse", &Ws::IsWaitingForResponse)
      .SetMethod("_stop", &Ws::Stop)
      .SetMethod("_goBack", &Ws::GoBack)
      .SetMethod("_goForward", &Ws::GoForward)
      .SetMethod("_goToOffset", &Ws::GoToOffset)
      .SetMethod("isCrashed", &Ws::IsCrashed)
      .SetMethod("_setUserAgent", &Ws::SetUserAgent)
      .SetMethod("_getUserAgent", &Ws::GetUserAgent)
      .SetProperty("userAgent", &Ws::GetUserAgent,
                   &Ws::SetUserAgent)
      .SetMethod("savePage", &Ws::SavePage)
      .SetMethod("openDevTools", &Ws::OpenDevTools)
      .SetMethod("closeDevTools", &Ws::CloseDevTools)
      .SetMethod("isDevToolsOpened", &Ws::IsDevToolsOpened)
      .SetMethod("isDevToolsFocused", &Ws::IsDevToolsFocused)
      .SetMethod("enableDeviceEmulation", &Ws::EnableDeviceEmulation)
      .SetMethod("disableDeviceEmulation", &Ws::DisableDeviceEmulation)
      .SetMethod("toggleDevTools", &Ws::ToggleDevTools)