### 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
return ID() == ontents->ID();
}
void WebContents::LoadURL(const GURL& url,
                          const gin_helper::Dictionary& options) {
  if (!url.is_valid() || url.spec().size() > url::kMaxURLChars) {
    Emit("did-fail-load", static_cast<int>(net::ERR_INVALID_URL),
         net::ErrorToShortString(net::ERR_INVALID_URL),
         url.possibly_invalid_spec(), true);
    return;
  }
  content::NavigationController::LoadURLParams params(url);
  if (!options.Get("httpReferrer", &params.referrer)) {
    GURL http_referrer;
    if (options.Get("httpReferrer", &http_referrer))
      params.referrer =
          content::Referrer(http_referrer.GetAsReferrer(),
                            network::mojom::ReferrerPolicy::kDefault);
  }
  std::string user_agent;
  if (options.Get("userAgent", &user_agent))
    ontents()->SetUserAgentOverride(
        blink::UserAgentOverride::UserAgentOnly(user_agent), false);
  std::string extra_headers;
  if (options.Get("extraHeaders", &extra_headers))
    params.extra_headers = extra_headers;
  scoped_refptr<network::ResourceRequestBody> body;
  if (options.Get("postData", &body)) {
    params.post_data = body;