Gardening: Mark multicol/pagination tests as flaky

BUG=361729
TBR=enne@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/257043002

git-svn-id: svn://svn.chromium.org/blink/trunk@172779 bbb929c8-8fbe-4397-9dbb-9b2b20218538