### 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
}
void Ws::Replace(const base::string16& word) {
  ontents()->Replace(word);
}
void Ws::ReplaceMisspelling(const base::string16& word) {
  ontents()->ReplaceMisspelling(word);
}
uint32_t Ws::FindInPage(gin_helper::Arguments* args) {
  base::string16 search_text;
  if (!args->GetNext(&search_text) || search_text.empty()) {
    args->ThrowError("Must provide a non-empty search content");
    return 0;
  }
  uint32_t request_id = GetNextRequestId();
  gin_helper::Dictionary dict;
  auto options = blink::mojom::FindOptions::New();
  if (args->GetNext(&dict)) {
    dict.Get("forward", &options->forward);
    dict.Get("matchCase", &options->match_case);
    dict.Get("findNext", &options->find_next);
  }
  ontents()->Find(request_id, search_text, std::move(options));
  return request_id;
}
void Ws::StopFindInPage(content::StopFindAction action) {
  ontents()->StopFinding(action);
}
void Ws::ShowDefinitionForSelection() {
#if defined(OS_MACOSX)
  auto* const view = ontents()->GetRenderWidgetHostView();