### 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
PrintViewManagerBasic class does not do null check.
  if (!ontents()) {
    if (print_callback)
      std::move(print_callback).Run(false, "failed");
    return;
  }
  std::u16string printer_name =
      device_name.empty() ? default_printer : device_name;
   If there are no valid printers available on the network, we bail.
  if (printer_name.empty() || !IsDeviceNameValid(printer_name)) {
    if (print_callback)
      std::move(print_callback).Run(false, "no valid printers available");
    return;
  }
  print_settings.SetStringKey(printing::kSettingDeviceName, printer_name);
  auto* print_view_manager =
      printing::PrintViewManagerBasic::FromWebContents(ontents());
  auto* focused_frame = ontents()->GetFocusedFrame();
  auto* rfh = focused_frame && focused_frame->HasSelection()
                  ? focused_frame
                  : ontents()->GetMainFrame();
  print_view_manager->PrintNow(rfh, silent, std::move(print_settings),
                               std::move(print_callback));
}
void WebContents::Print(gin::Arguments* args) {
  gin_helper::Dictionary options =
      gin::Dictionary::CreateEmpty(args->isolate());
  base::Value settings(base::Value::Type::DICTIONARY);
  if (args->Length() >= 1 && !args->GetNext(&options)) {
    gin_helper::ErrorThrower(args->isolate())