### 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
#else
    if (!base::PathService::Get(content::CHILD_PROCESS_EXE, &child_path)) {
      CHECK(false) << "Unable to get child process binary name.";
    }
    SCOPED_CRASH_KEY_STRING256("ChildProcess", "child_process_exe",
                               child_path.AsUTF8Unsafe());
    SCOPED_CRASH_KEY_STRING256("ChildProcess", "program",
                               program.AsUTF8Unsafe());
    CHECK_EQ(program, child_path);
#endif
  }
  std::string process_type =
      command_line->GetSwitchValueASCII(::switches::kProcessType);
#if defined(OS_LINUX)
  bool enable_crash_reporter = false;
  enable_crash_reporter = breakpad::IsCrashReporterEnabled();
  if (enable_crash_reporter) {
    std::string switch_value =
        api::crash_reporter::GetClientId() + ",no_channel";
    command_line->AppendSwitchASCII(::switches::kEnableCrashReporter,
                                    switch_value);
    for (const auto& pair : api::crash_reporter::GetGlobalCrashKeys()) {
      if (!switch_value.empty())
        switch_value += ",";
      switch_value += pair.first;
      switch_value += "=";
      switch_value += pair.second;
    }
    command_line->AppendSwitchASCII(switches::kGlobalCrashKeys, switch_value);
  }