Convert fast/multicol/float-paginate to reftest.

Added an explicit column-gap, used column-count rather than column-width,
and removed some lines to make the test fit better in content_shell.

R=mstensho@opera.com
BUG=386567

Review URL: https://codereview.chromium.org/695283002

git-svn-id: svn://svn.chromium.org/blink/trunk@184868 bbb929c8-8fbe-4397-9dbb-9b2b20218538