fix(form-field): inconsistent border width across outline gap in Chrome

[kherock]

Closes #10710 (You can see screenshots there).

Chrome render borders that have non-integer widths differently when a border-radius is set. This adds an imperceptibly small border radius to the outline gap so that it renders the same as the outside borders.

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[kherock]

CLA is signed!

[googlebot]

CLAs look good, thanks!

[kherock]

Sorry for all the noise on CI, clearly I need some rest. I think this should be ready to be looked at!

[mmalerba]

Thanks for the PR. I'll need to bring up some demos and test across different browsers

[mmalerba]

Looks great on the browsers I tested!

[mmalerba]

Did you read about this somewhere, or just figured it out through experimentation? If you read about it, it would be nice to include a link for reference

[kherock]

Nope, this was just through experimentation! Unfortunately I couldn't find any reported issue related to this. I don't imagine this scenario comes up too often.

[kherock]

I just tweaked my comment to be a bit more accurate and updated the issue reference to be a URL rather than just the issue number.

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