Re-enable test fast/multicol/cell-shrinkback.html and make it pass.

Preferred min/max widths are rounded up for table cells, but not for other
types of shrink-to-fit containers, so using a table cell in the test and
an abspos in the ref doesn't necessarily result in identical rendering.

Specify a fixed width on the containers instead, so that the test isn't
affected by this problem. The problem has nothing to do with multicol.

BUG=397257
R=pkasting@chromium.org

Review URL: https://codereview.chromium.org/501853003

git-svn-id: svn://svn.chromium.org/blink/trunk@180999 bbb929c8-8fbe-4397-9dbb-9b2b20218538