### 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
gfx::Rect rect;
  gin_helper::Promise<gfx::Image> promise(args->isolate());
  v8::Local<v8::Promise> handle = promise.GetHandle();
   get rect arguments if they exist
  args->GetNext(&rect);
  auto* const view = ontents()->GetRenderWidgetHostView();
  if (!view) {
    promise.Resolve(gfx::Image());
    return handle;
  }
#if !defined(OS_MAC)
   If the view's renderer is suspended this may fail on Windows/Linux -
   bail if so. See CopyFromSurface in
   content/public/browser/render_widget_host_view.h.
  auto* rfh = ontents()->GetMainFrame();
  if (rfh &&
      rfh->GetVisibilityState() == blink::mojom::PageVisibilityState::kHidden) {
    promise.Resolve(gfx::Image());
    return handle;
  }
#endif   defined(OS_MAC)
   Capture full page if user doesn't specify a |rect|.
  const gfx::Size view_size =
      rect.IsEmpty() ? view->GetViewBounds().size() : rect.size();
   By default, the requested bitmap size is the view size in screen
   coordinates.  However, if there's more pixel detail available on the
   current system, increase the requested bitmap size to capture it all.