### 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_browser_window.cc
gin::Handle<WebContentsView> web_contents_view =
      WebContentsView::Create(isolate, web_preferences);
  DCHECK(web_contents_view.get());
  
  gin::Handle<WebContents> web_contents =
      web_contents_view->GetWebContents(isolate);
  web_contents_.Reset(isolate, web_contents.ToV8());
  api_web_contents_ = web_contents->GetWeakPtr();
  api_web_contents_->AddObserver(this);
  Observe(api_web_contents_->web_contents());
  
  gin_helper::Dictionary(isolate, web_contents.ToV8().As<v8::Object>())
      .Set("browserWindowOptions", options);
  
  web_contents->SetOwnerWindow(window());
  auto* host = web_contents->web_contents()->GetRenderViewHost();
  if (host)
    host->GetWidget()->AddInputEventObserver(this);
  InitWithArgs(args);
  
  SetContentView(gin::CreateHandle<View>(isolate, web_contents_view.get()));
#if defined(OS_MAC)
  OverrideNSWindowContentView(
      web_contents->inspectable_web_contents()->GetView());
#endif
  
  window()->InitFromOptions(options);
}
BrowserWindow::~BrowserWindow() {