CSS: Don't read styles.position in the width/height cssHook unless necessary

CSS: Don't read styles.position in the width/height cssHook unless necessary

Current width/height cssHook reads the computed position style even if not
necessary as the browser passes the scrollboxSize support test. That has been
changed.

This commit also makes the scrollboxSize support test in line with all others
(i.e. only return true or false) and changes the variable name in the hook
to make the code clearer.

Fixes gh-4185
Closes gh-4187