### 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
window_unresponsive_closure_.Cancel();
  Emit("responsive");
}
void BrowserWindow::OnDraggableRegionsUpdated(
    const std::vector<mojom::DraggableRegionPtr>& regions) {
  UpdateDraggableRegions(regions);
}
void BrowserWindow::OnSetContentBounds(const gfx::Rect& rect) {
  
  
  window()->SetBounds(rect, false);
}
void BrowserWindow::OnActivateContents() {
  
#if !defined(OS_MAC)
  if (IsMenuBarAutoHide() && IsMenuBarVisible())
    window()->SetMenuBarVisibility(false);
#endif
}
void BrowserWindow::OnPageTitleUpdated(const std::u16string& title,
                                       bool explicit_set) {
  
  auto self = GetWeakPtr();
  if (!Emit("page-title-updated", title, explicit_set)) {
    
    if (self && !IsDestroyed())
      SetTitle(base::UTF16ToUTF8(title));
  }
}
void BrowserWindow::RequestPreferredWidth(int* width) {