Fixes gh-3589

Summary

Per spec, a scrollbar gutter is excluded from content dimensions. But we want to include it in .innerWidth/.innerHeight/.outerWidth/.outerHeight, and therefore must calculate it from other data.

I also renamed functions and variables for clarity (but kept that work in its own commit for easier reviewing).

It's surprisingly big:

   raw     gz Sizes
269640  80000 dist/jquery.js
 86921  30123 dist/jquery.min.js

   raw     gz Compared to master @ b3defccdc0ca93b49647e79df5e70fb933af679c
  +708   +209 dist/jquery.js
  +139    +36 dist/jquery.min.js

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