### 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
content::WebContents* source,
    std::unique_ptr<content::WebContents> new_contents,
    const GURL& target_url,
    WindowOpenDisposition disposition,
    const gfx::Rect& initial_rect,
    bool user_gesture,
    bool* was_blocked) {
  auto* tracker = ChildWebContentsTracker::FromWebContents(new_contents.get());
  DCHECK(tracker);
  v8::Isolate* isolate = JavascriptEnvironment::GetIsolate();
  v8::Locker locker(isolate);
  v8::HandleScope handle_scope(isolate);
  auto api_ontents =
      CreateAndTake(isolate, std::move(new_contents), Type::kBrowserWindow);
  
  
  
  
  auto* frame = api_ontents->MainFrame();
  if (frame) {
    api_ontents->HandleNewRenderFrame(frame);
  }
  if (Emit("-add-new-contents", api_ontents, disposition, user_gesture,
           initial_rect.x(), initial_rect.y(), initial_rect.width(),
           initial_rect.height(), tracker->url, tracker->frame_name,
           tracker->referrer, tracker->raw_features, tracker->body)) {
    api_ontents->Destroy();
  }
}
content::WebConbContents::OpenURLFromTab(