### 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
params.load_type = content::NavigationController::LOAD_TYPE_HTTP_POST;
  }
  GURL base_url_for_data_url;
  if (options.Get("baseURLForDataURL", &base_url_for_data_url)) {
    params.base_url_for_data_url = base_url_for_data_url;
    params.load_type = content::NavigationController::LOAD_TYPE_DATA;
  }
  bool reload_ignoring_cache = false;
  if (options.Get("reloadIgnoringCache", &reload_ignoring_cache) &&
      reload_ignoring_cache) {
    params.reload_type = content::ReloadType::BYPASSING_CACHE;
  }
   Calling LoadURLWithParams() can trigger JS which destroys |this|.
  auto weak_this = GetWeakPtr();
  params.transition_type = ui::PAGE_TRANSITION_TYPED;
  params.should_clear_history_list = true;
  params.override_user_agent = content::NavigationController::UA_OVERRIDE_TRUE;
   Discord non-committed entries to ensure that we don't re-use a pending
   entry
  ontents()->GetController().DiscardNonCommittedEntries();
  ontents()->GetController().LoadURLWithParams(params);
   ⚠️WARNING!⚠️
   LoadURLWithParams() triggers JS events which can call destroy() on |this|.
   It's not safe to assume that |this| points to valid memory at this point.
  if (!weak_this || !ontents())
    return;
   Required to make beforeunload handler work.
  NotifyUserActivation();
}
void WebContents::DownloadURL(const GURL& url) {