Rebaseline clear-scroll-parent layout test on mac

This platform had a different graphics layer tree topology, but the
difference is benign.

TBR=loislo@chromium.org
BUG=298039

Review URL: https://codereview.chromium.org/286143002

git-svn-id: svn://svn.chromium.org/blink/trunk@174172 bbb929c8-8fbe-4397-9dbb-9b2b20218538