Mark autoheight-regions-mark.html as flaky

TBR=toyoshim@chromium.org
BUG=304993

Review URL: https://codereview.chromium.org/26384002

git-svn-id: svn://svn.chromium.org/blink/trunk@159059 bbb929c8-8fbe-4397-9dbb-9b2b20218538