### 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
std::u16string search_text;
  if (!args->GetNext(&search_text) || search_text.empty()) {
    gin_helper::ErrorThrower(args->isolate())
        .ThrowError("Must provide a non-empty search content");
    return 0;
  }
  uint32_t request_id = ++find_in_page_request_id_;
  gin_helper::Dictionary dict;
  auto options = blink::mojom::FindOptions::New();
  if (args->GetNext(&dict)) {
    dict.Get("forward", &options->forward);
    dict.Get("matchCase", &options->match_case);
    dict.Get("findNext", &options->new_session);
  }
  ontents()->Find(request_id, search_text, std::move(options));
  return request_id;
}
void WebContents::StopFindInPage(content::StopFindAction action) {
  ontents()->StopFinding(action);
}
void WebContents::ShowDefinitionForSelection() {
#if defined(OS_MAC)
  auto* const view = ontents()->GetRenderWidgetHostView();
  if (view)
    view->ShowDefinitionForSelection();
#endif
}
void WebContents::CopyImageAt(int x, int y) {