### 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:** 11.2.1
* **Operating System:** Windows 10

### Expected Behavior
I expect that `will-prevent-unload` will fire when closing a BrowserView with a page that cancels the `beforeunload` event

### Actual Behavior
`will-prevent-unload` never fires. It only fires for BrowserWindows.

### To Reproduce
1. start main.js
2. in the devtools, execute: `onbeforeunload = (e) => { e.preventDefault(); e.returnValue = false; }`
3. Close the window.
4. The window does close, but the `will-prevent-unload` handler never executes

```
const { app, BrowserWindow, BrowserView } = require('electron')

async function createWindow () {
  const win = new BrowserWindow();
  const view = new BrowserView();

  view.setBounds({x: 0, y: 0, width: 600, height: 600});
  win.setBrowserView(view);

  await view.webContents.loadURL('https://google.com')
  view.webContents.openDevTools();

  view.webContents.on("will-prevent-unload", (e) => {
    e.preventDefault();
  });
}

app.whenReady().then(createWindow)

app.on('window-all-closed', () => {
  if (process.platform !== 'darwin') {
    app.quit()
  }
});
```

Related (if not a dupe) of [this issue](https://github.com/electron/electron/issues/19194)
shell/browser/api/electron_api_web_contents.cc
printing::mojom::DuplexMode::kSimplex;
  options.Get("duplexMode", &duplex_mode);
  settings.SetIntKey(printing::kSettingDuplexMode,
                     static_cast<int>(duplex_mode));
   We've already done necessary parameter sanitization at the
   JS level, so we can simply pass this through.
  base::Value media_size(base::Value::Type::DICTIONARY);
  if (options.Get("mediaSize", &media_size))
    settings.SetKey(printing::kSettingMediaSize, std::move(media_size));
   Set custom dots per inch (dpi)
  gin_helper::Dictionary dpi_settings;
  int dpi = 72;
  if (options.Get("dpi", &dpi_settings)) {
    int horizontal = 72;
    dpi_settings.Get("horizontal", &horizontal);
    settings.SetIntKey(printing::kSettingDpiHorizontal, horizontal);
    int vertical = 72;
    dpi_settings.Get("vertical", &vertical);
    settings.SetIntKey(printing::kSettingDpiVertical, vertical);
  } else {
    settings.SetIntKey(printing::kSettingDpiHorizontal, dpi);
    settings.SetIntKey(printing::kSettingDpiVertical, dpi);
  }
  print_task_runner_->PostTaskAndReplyWithResult(
      FROM_HERE, base::BindOnce(&GetDefaultPrinterAsync),
      base::BindOnce(&WebContents::OnGetDefaultPrinter,
                     weak_factory_.GetWeakPtr(), std::move(settings),
                     std::move(callback), device_name, silent));
}
v8::Local<v8::Promise> WebContents::PrintToPDF(base::DictionaryValue settings) {