Fix border-radius of commonFormBottomWrapper

[luixxiul]

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

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).

[srirambv]

Looks good to me. 👍

[cezaraugusto]

++