Fix border-radius of commonFormBottomWrapper
Fix border-radius of commonFormBottomWrapper

Closes #8649

Auditors:

Test Plan:
1. Open about:styles
2. Click "commonForm"
3. Make sure the dialog has border-radius on bottom right and left

timborden/browser-laptop,timborden/browser-laptop,diasdavid/browser-laptop,luixxiul/browser-laptop,luixxiul/browser-laptop,diasdavid/browser-laptop,timborden/browser-laptop,timborden/browser-laptop,luixxiul/browser-laptop,luixxiul/browser-laptop,diasdavid/browser-laptop,diasdavid/browser-laptop