### 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
ElectronBrowserClient::CreateBrowserMainParts(
    const content::MainFunctionParams& params) {
  auto browser_main_parts = std::make_unique<ElectronBrowserMainParts>(params);
#if defined(OS_MAC)
  browser_main_parts_ = browser_main_parts.get();
#endif
  return browser_main_parts;
}
void ElectronBrowserClient::WebNotificationAllowed(
    content::RenderFrameHost* rfh,
    base::OnceCallback<void(bool, bool)> callback) {
  content::WebContents* web_contents =
      content::WebContents::FromRenderFrameHost(rfh);
  if (!web_contents) {
    std::move(callback).Run(false, false);
    return;
  }
  auto* permission_helper =
      WebContentsPermissionHelper::FromWebContents(web_contents);
  if (!permission_helper) {
    std::move(callback).Run(false, false);
    return;
  }
  permission_helper->RequestWebNotificationPermission(
      base::BindOnce(std::move(callback), web_contents->IsAudioMuted()));
}
void ElectronBrowserClient::RenderProcessHostDestroyed(