### 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
base::FilePath path;
  if (base::PathService::Get(base::DIR_HOME, &path))
    path = path.Append(FILE_PATH_LITERAL("Downloads"));
  return path;
}
scoped_refptr<network::SharedURLLoaderFactory>
ElectronBrowserClient::GetSystemSharedURLLoaderFactory() {
  if (!g_browser_process)
    return nullptr;
  return g_browser_process->shared_url_loader_factory();
}
void ElectronBrowserClient::OnNetworkServiceCreated(
    network::mojom::NetworkService* network_service) {
  if (!g_browser_process)
    return;
  g_browser_process->system_network_context_manager()->OnNetworkServiceCreated(
      network_service);
}
std::vector<base::FilePath>
ElectronBrowserClient::GetNetworkContextsParentDirectory() {
  base::FilePath user_data_dir;
  base::PathService::Get(chrome::DIR_USER_DATA, &user_data_dir);
  DCHECK(!user_data_dir.empty());
  return {user_data_dir};
}
std::string ElectronBrowserClient::GetProduct() {