### 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
}
    if (IsGuest())
      Emit("load-commit", url, is_main_frame);
  } else {
    auto url = navigation_handle->GetURL();
    int code = navigation_handle->GetNetErrorCode();
    auto description = net::ErrorToShortString(code);
    Emit("did-fail-provisional-load", code, description, url, is_main_frame,
         frame_process_id, frame_routing_id);
     Do not emit "did-fail-load" for canceled requests.
    if (code != net::ERR_ABORTED) {
      EmitWarning(
          node::Environment::GetCurrent(JavascriptEnvironment::GetIsolate()),
          "Failed to load URL: " + url.possibly_invalid_spec() +
              " with error: " + description,
          "electron");
      Emit("did-fail-load", code, description, url, is_main_frame,
           frame_process_id, frame_routing_id);
    }
  }
  content::NavigationEntry* entry = navigation_handle->GetNavigationEntry();
   This check is needed due to an issue in Chromium
   Check the Chromium issue to keep updated:
   https:bugs.chromium.org/p/chromium/issues/detail?id=1178663
   If a history entry has been made and the forward/back call has been made,
   proceed with setting the new title
  if (entry && (entry->GetTransitionType() & ui::PAGE_TRANSITION_FORWARD_BACK))
    WebContents::TitleWasSet(entry);
}
void WebContents::TitleWasSet(content::NavigationEntry* entry) {