When running Perf tryjob on all-win platforms, the config generated by trybot_command for win32 is incorrect.

https://build.chromium.org/p/tryserver.chromium.perf/builders/win_perf_bisect/builds/6380/steps/load%20config/logs/json.output

Incorrect config
{
  "command": "python tools\\perf\\run_benchmark --browser=release_x64 page_cycler.typical_25", 
  "max_time_minutes": "120", 
  "repeat_count": "1", 
  "target_arch": "x64", 
  "truncate_percent": "0"
}

Expected config
{
  "command": "python tools\\perf\\run_benchmark --browser=release page_cycler.typical_25", 
  "max_time_minutes": "120", 
  "repeat_count": "1", 
  "target_arch": "ia32", 
  "truncate_percent": "0"
}