[New Multicolumn] Remove the term "balancing" from some member names.

This is in line with what it's called in RenderMultiColumnFlowThread()
(recalculateColumnHeights()), and soon (once we have support for multiple
column sets) this isn't going to be strictly about balancing anyway.

Review URL: https://codereview.chromium.org/264773018

git-svn-id: svn://svn.chromium.org/blink/trunk@173368 bbb929c8-8fbe-4397-9dbb-9b2b20218538