[New Multicolumn] Correct paint rectangle calculation for compositing.

This allows for enabling composited layers inside flow threads, since it's
not completely broken anymore.

BUG=359877

Review URL: https://codereview.chromium.org/625903004

git-svn-id: svn://svn.chromium.org/blink/trunk@183437 bbb929c8-8fbe-4397-9dbb-9b2b20218538