### 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
// For webview only #4 will happen, for BrowserWindow both #1 and #3 may
// happen. The #2 should never happen for ws, because webview is
// managed by GuestViewManager, and BrowserWindow's ws is managed
// by api::BrowserWindow.
// For #1, the destructor will do the cleanup work and we only need to make
// sure "destroyed" event is emitted. For #3, the content::Ws will
// be destroyed on close, and WsDestroyed would be called for it, so
// we need to make sure the api::Ws is also deleted.
// For #4, the Ws will be destroyed by embedder.
void Ws::WsDestroyed() {
  // Cleanup relationships with other parts.
  RemoveFromWeakMap();
  // We can not call Destroy here because we need to call Emit first, but we
  // also do not want any method to be used, so just mark as destroyed here.
  MarkDestroyed();
  Emit("destroyed");
  // For guest view based on OOPIF, the Ws is released by the embedder
  // frame, and we need to clear the reference to the memory.
  if (IsGuest() && managed_ontents()) {
    managed_ontents()->ReleaseWs();
    ResetManagedWs(false);
  }
  // Destroy the native class in next tick.
  base::ThreadTaskRunnerHandle::Get()->PostTask(FROM_HERE, GetDestroyClosure());
}
void Ws::NavigationEntryCommitted(
    const content::LoadCommittedDetails& details) {
  Emit("navigation-entry-committed", details.entry->GetURL(),
       details.is_same_document, details.did_replace_entry);
}