Mark fast/repaint/selection-gap-overflow-scroll-2.html as flaky.

BUG=345767
TBR=acolwell@chromium.org

Review URL: https://codereview.chromium.org/175563002

git-svn-id: svn://svn.chromium.org/blink/trunk@167605 bbb929c8-8fbe-4397-9dbb-9b2b20218538