### 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
load_url_params.frame_tree_node_id = params.frame_tree_node_id;
  load_url_params.redirect_chain = params.redirect_chain;
  load_url_params.has_user_gesture = params.user_gesture;
  load_url_params.blob_url_loader_factory = params.blob_url_loader_factory;
  load_url_params.href_translate = params.href_translate;
  load_url_params.reload_type = params.reload_type;
  if (params.post_data) {
    load_url_params.load_type =
        content::NavigationController::LOAD_TYPE_HTTP_POST;
    load_url_params.post_data = params.post_data;
  }
  source->GetController().LoadURLWithParams(load_url_params);
  return source;
}
void WebContents::BeforeUnloadFired(content::WebContents* tab,
                                    bool proceed,
                                    bool* proceed_to_fire_unload) {
  if (type_ == Type::kBrowserWindow || type_ == Type::kOffScreen)
    *proceed_to_fire_unload = proceed;
  else
    *proceed_to_fire_unload = true;
  
  Emit("before-unload-fired", proceed);
}
void WebContents::SetContentsBounds(content::WebContents* source,
                                    const gfx::Rect& rect) {
  for (ExtendedWebContentsObserver& observer : observers_)
    observer.OnSetContentBounds(rect);
}
void WebContents::CloseContents(content::WebContents* source) {