CSS: Avoid unit-conversion interference from CSS upper bounds

[gibson042]

Summary

Because Firefox getComputedStyle reports post-calculation used values (and therefore arbitrarily high margin values all result in the same used value), we need to iterate towards something else when converting units.

Fixes gh-2144

Checklist

• All authors have signed the CLA at https://cla.js.foundation/jquery/jquery
• New tests have been added to show the fix or feature works
• Grunt build and unit tests pass locally with these changes
• If needed, a docs issue/PR was created at https://github.com/jquery/api.jquery.com

[mention-bot]

@gibson042, thanks for your PR! By analyzing the history of the files in this pull request, we identified @mr21, @markelog and @gnarf to be potential reviewers.

[timmywil]

LGTM

[Krinkle]

@timmywil @gibson042 This broke the build for iOS and Safari.

http://swarm.jquery.org/project/jquery

iOS 7.0, iOS 8.3: http://swarm.jquery.org/result/2416819

iOS 9, iOS 10, Safari 9, Safari 10: http://swarm.jquery.org/result/2403960

[gibson042]

I know; the fix is awaiting review at #3778 .

[Krinkle]

Thanks, all green now!