### 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](https://www.github.com/electron/electron/issues) for a feature request that matches the one I want to file, without success.

### Electron Version

main

### What operating system are you using?

All OSes.

### What arch are you using?

All archs.

### Last Known Working Electron version

N/A

### Expected Behavior

[`ElectronBrowserClient::GetDefaultDownloadDirectory()`](https://github.com/electron/electron/blob/main/shell/browser/electron_browser_client.cc#L1013-L1019) is hardcoded to return `~/Downloads`. However, this might not be the actual downloads directory path (eg: if the user has changed it or on OSes which translate user directory names).

The proper way to get the current user's downloads directory is OS-dependent:
- On Linux it is set in the env var `$XDG_DOWNLOAD_DIR` 
- On Windows it is returned by [`SHGetKnownFolderPath`](https://docs.microsoft.com/en-us/windows/win32/api/shlobj_core/nf-shlobj_core-shgetknownfolderpath) passing in `FOLDERID_Downloads`
- On MacOS it is returned by [`URLForDirectory`](https://developer.apple.com/documentation/foundation/nsfilemanager/1407693-urlfordirectory) passing in `NSDownloadsDirectory`

### Current Behavior

`~/Downloads` is [hardcoded](https://github.com/electron/electron/blob/main/shell/browser/electron_browser_client.cc#L1013-L1019).

shell/browser/electron_browser_client.cc
if (!bluetooth_delegate_)
    bluetooth_delegate_ = std::make_unique<ElectronBluetoothDelegate>();
  return bluetooth_delegate_.get();
}
void BindBadgeServiceForServiceWorker(
    const content::ServiceWorkerVersionBaseInfo& info,
    mojo::PendingReceiver<blink::mojom::BadgeService> receiver) {
  DCHECK_CURRENTLY_ON(content::BrowserThread::UI);
  content::RenderProcessHost* render_process_host =
      content::RenderProcessHost::FromID(info.process_id);
  if (!render_process_host)
    return;
  badging::BadgeManager::BindServiceWorkerReceiver(
      render_process_host, info.scope, std::move(receiver));
}
void ElectronBrowserClient::RegisterBrowserInterfaceBindersForServiceWorker(
    mojo::BinderMapWithContext<const content::ServiceWorkerVersionBaseInfo&>*
        map) {
  map->Add<blink::mojom::BadgeService>(
      base::BindRepeating(&BindBadgeServiceForServiceWorker));
}
device::GeolocationManager* ElectronBrowserClient::GetGeolocationManager() {
#if defined(OS_MAC)
  return browser_main_parts_->GetGeolocationManager();
#else
  return nullptr;
#endif
}
}