Add rounded corners on CommandBar overflow menu

[kaiguo]

Description

Adding 4px rounded corners for CommandBar overflow menu.

Motivation and Context

Fixes #1431.

How Has This Been Tested?

Added visual tree verification tests for CommandBar overflow menu and CommandBar itself.

Screenshots (if appropriate):

[mdtauk]

@kaiguo In that example image, shouldn't the bottom two corners have a 0 CornerRadius, as it is extending from the bottom bar.

Also does this take into account the full width flyout, when in a narrow window/phone?

[kmahone]

[kaiguo]

@kaiguo Kai Guo FTE In that example image, shouldn't the bottom two corners have a 0 CornerRadius, as it is extending from the bottom bar.

Also does this take into account the full width flyout, when in a narrow window/phone?

I think @chigy reviewed with design team and we decided to make it 4px for all corners, #1431 (comment).

And I believe it applies to full width flyouts too.

[chigy]

@kaiguo Kai Guo FTE In that example image, shouldn't the bottom two corners have a 0 CornerRadius, as it is extending from the bottom bar.
Also does this take into account the full width flyout, when in a narrow window/phone?

I think @chigy reviewed with design team and we decided to make it 4px for all corners, #1431 (comment).

And I believe it applies to full width flyouts too.

That is correct. The image shared is what we expect the design to be.