ComboBox box-sizing broken the bottom border

[stephenzsy]

Bug Report

• Package version(s): 5.46
• Browser and OS versions: Chrome 64

Priorities and help requested (not applicable if asking question):

Are you willing to submit a PR to fix? No

Requested priority: Blocking

Products/sites affected: (if applicable)

Describe the issue:

looks this change 5c71bd9 changed the box-sizing from "content-box" to "border-box" which causes the rending broken.

Actual behavior:

Expected behavior:

If applicable, please provide a codepen repro:

[stephenzsy]

It is visible on the office fabric page now

[lynamemi]

We changed to border-box to align to TextField and Dropdown and to better support high contrast mode styles. I'll look into where that overlap is coming from.