[New Multicolumn] Let updatePagination() do its job also when composited.

Otherwise, layers inside multicol will become non-paginated if relaid out
after initial paint.

BUG=359877
R=jchaffraix@chromium.org

Review URL: https://codereview.chromium.org/660793002

git-svn-id: svn://svn.chromium.org/blink/trunk@184012 bbb929c8-8fbe-4397-9dbb-9b2b20218538