### 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
<< "Can't take ownership of a remote WebContents";
  auto session = Session::CreateFrom(isolate, GetBrowserContext());
  session_.Reset(isolate, session.ToV8());
  InitWithSessionAndOptions(isolate, std::move(ontents), session,
                            gin::Dictionary::CreateEmpty(isolate));
}
WebContents::WebContents(v8::Isolate* isolate,
                         const gin_helper::Dictionary& options)
    : id_(GetAllWebContents().Add(this)),
      devtools_file_system_indexer_(new DevToolsFileSystemIndexer),
      file_task_runner_(
          base::ThreadPool::CreateSequencedTaskRunner({base::MayBlock()}))
#if BUILDFLAG(ENABLE_PRINTING)
      ,
      print_task_runner_(CreatePrinterHandlerTaskRunner())
#endif
{
  
  options.Get("backgroundThrottling", &background_throttling_);
  
  options.Get("type", &type_);
#if BUILDFLAG(ENABLE_OSR)
  bool b = false;
  if (options.Get(options::kOffscreen, &b) && b)
    type_ = Type::kOffScreen;
#endif
  
  options.Get("embedder", &embedder_);
  
  options.Get("devTools", &enable_devtools_);