### 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:** 5.0.7
* **Operating System:** Windows 10

### Expected Behavior
When I navigate a `BrowserView` to a page that has `beforeunload` and `unload` handlers, I expect that closing the window will cause them to fire.

### Actual Behavior
They do not fire when closing the window.

Notably, if I don't use `BrowserView` and just navigate the `BrowserWindow` to the page, those handlers will execute.

### To Reproduce

`npm start` the code below (in main.js). In the devtools that opens up, run `window.onbeforeunload = () => true;`, then close the window. In the working case, the window will not close. In the broken case that uses `BrowserView`, it will.

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

function brokenCase() {
	const mainWindow = new BrowserWindow();
	const view = new BrowserView();
	mainWindow.setBrowserView(view);
	view.webContents.loadURL("https://google.com");
	view.webContents.openDevTools();
}

function workingCase() {
	const mainWindow = new BrowserWindow();
	mainWindow.webContents.loadURL("https://google.com");
	mainWindow.webContents.openDevTools();
}

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

Maybe related to [this comment](https://github.com/electron/electron/issues/19194#issuecomment-544407723)
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) {