Change clip rect computation for multicol

Change the way clip rects are determined for multicol by using enclosing
IntRect instead of pixelSnappedIntRect to ensure that the colum contents
fully fits within the clip rect.

R=leviw@chromium.org, mstensho@opera.com

Review URL: https://codereview.chromium.org/300843010

git-svn-id: svn://svn.chromium.org/blink/trunk@174983 bbb929c8-8fbe-4397-9dbb-9b2b20218538