Cleanup: Replace adjustForColumns() / offsetForColumns() with columnOffset().

Most of the former adjustForColumns() call sites now look simpler,
since they call a method that returns the offset instead of using
pass-by-reference.

R=ojan@chromium.org

Review URL: https://codereview.chromium.org/259993006

git-svn-id: svn://svn.chromium.org/blink/trunk@172853 bbb929c8-8fbe-4397-9dbb-9b2b20218538