### 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
const ui::Cursor& cursor = webcursor.cursor();
  if (cursor.type() == ui::mojom::CursorType::kCustom) {
    Emit("cursor-changed", CursorTypeToString(cursor),
         gfx::Image::CreateFrom1xBitmap(cursor.custom_bitmap()),
         cursor.image_scale_factor(),
         gfx::Size(cursor.custom_bitmap().width(),
                   cursor.custom_bitmap().height()),
         cursor.custom_hotspot());
  } else {
    Emit("cursor-changed", CursorTypeToString(cursor));
  }
}
bool WebContents::IsGuest() const {
  return type_ == Type::kWebView;
}
void WebContents::AttachToIframe(content::WebContents* embedder_ontents,
                                 int embedder_frame_id) {
  attached_ = true;
  if (guest_delegate_)
    guest_delegate_->AttachToIframe(embedder_ontents, embedder_frame_id);
}
bool WebContents::IsOffScreen() const {
#if BUILDFLAG(ENABLE_OSR)
  return type_ == Type::kOffScreen;
#else
  return false;
#endif
}
#if BUILDFLAG(ENABLE_OSR)
void WebContents::OnPaint(const gfx::Rect& dirty_rect, const SkBitmap& bitmap) {