### 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
content::WebContents::FromRenderFrameHost(render_frame_host);
  if (!web_contents)
    return;
  const GURL& site = render_frame_host->GetSiteInstance()->GetSiteURL();
  if (!site.SchemeIs(extensions::kExtensionScheme))
    return;
  content::BrowserContext* browser_context =
      render_frame_host->GetProcess()->GetBrowserContext();
  auto* extension = extensions::ExtensionRegistry::Get(browser_context)
                        ->enabled_extensions()
                        .GetByID(site.host());
  if (!extension)
    return;
  extensions::ExtensionsBrowserClient::Get()
      ->RegisterBrowserInterfaceBindersForFrame(map, render_frame_host,
                                                extension);
#endif
}
#if defined(OS_LINUX)
void ElectronBrowserClient::GetAdditionalMappedFilesForChildProcess(
    const base::CommandLine& command_line,
    int child_process_id,
    content::PosixFileDescriptorInfo* mappings) {
  int crash_signal_fd = GetCrashSignalFD(command_line);
  if (crash_signal_fd >= 0) {
    mappings->Share(kCrashDumpSignal, crash_signal_fd);
  }
}
#endif
std::unique_ptr<content::LoginDelegate>