### 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
auto it = devtools_indexing_jobs_.find(request_id);
  if (it == devtools_indexing_jobs_.end())
    return;
  it->second->Stop();
  devtools_indexing_jobs_.erase(it);
}
void WebContents::DevToolsSearchInPath(int request_id,
                                       const std::string& file_system_path,
                                       const std::string& query) {
  if (!IsDevToolsFileSystemAdded(GetDevToolsWebContents(), file_system_path)) {
    OnDevToolsSearchCompleted(request_id, file_system_path,
                              std::vector<std::string>());
    return;
  }
  devtools_file_system_indexer_->SearchInPath(
      file_system_path, query,
      base::BindRepeating(&WebContents::OnDevToolsSearchCompleted,
                          weak_factory_.GetWeakPtr(), request_id,
                          file_system_path));
}
#if defined(TOOLKIT_VIEWS) && !defined(OS_MAC)
gfx::ImageSkia WebContents::GetDevToolsWindowIcon() {
  if (!owner_window())
    return gfx::ImageSkia();
  return owner_window()->GetWindowAppIcon();
}
#endif
#if defined(OS_LINUX)
void WebContents::GetDevToolsWindowWMClass(std::string* name,