### 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:** 11.2.1
* **Operating System:** Windows 10

### Expected Behavior
I expect that `will-prevent-unload` will fire when closing a BrowserView with a page that cancels the `beforeunload` event

### Actual Behavior
`will-prevent-unload` never fires. It only fires for BrowserWindows.

### To Reproduce
1. start main.js
2. in the devtools, execute: `onbeforeunload = (e) => { e.preventDefault(); e.returnValue = false; }`
3. Close the window.
4. The window does close, but the `will-prevent-unload` handler never executes

```
const { app, BrowserWindow, BrowserView } = require('electron')

async function createWindow () {
  const win = new BrowserWindow();
  const view = new BrowserView();

  view.setBounds({x: 0, y: 0, width: 600, height: 600});
  win.setBrowserView(view);

  await view.webContents.loadURL('https://google.com')
  view.webContents.openDevTools();

  view.webContents.on("will-prevent-unload", (e) => {
    e.preventDefault();
  });
}

app.whenReady().then(createWindow)

app.on('window-all-closed', () => {
  if (process.platform !== 'darwin') {
    app.quit()
  }
});
```

Related (if not a dupe) of [this issue](https://github.com/electron/electron/issues/19194)
shell/browser/api/electron_api_web_contents.cc
ElectronBrowserContext* browser_context,
                                      bool is_guest) {
  browser_context_ = browser_context;
  ontents->SetDelegate(this);
#if BUILDFLAG(ENABLE_PRINTING)
  PrintPreviewMessageHandler::CreateForWebContents(ontents);
  printing::PrintViewManagerBasic::CreateForWebContents(ontents);
  printing::CreateCompositeClientIfNeeded(ontents,
                                          browser_context->GetUserAgent());
#endif
#if BUILDFLAG(ENABLE_PDF_VIEWER)
  pdf::PDFWebContentsHelper::CreateForWebContentsWithClient(
      ontents, std::make_unique<ElectronPDFWebContentsHelperClient>());
#endif
  
  auto* web_preferences = WebContentsPreferences::From(ontents);
  offscreen_ =
      web_preferences && web_preferences->IsEnabled(options::kOffscreen);
  
  inspectable_ontents_.reset(new InspectableWebContents(
      ontents, browser_context->prefs(), is_guest));
  inspectable_ontents_->SetDelegate(this);
}
WebContents::~WebContents() {