Mark editing/selection/4960116.html as flaky

Unreviewed gardening.

BUG=341682
TBR=abarth@chromium.org

Review URL: https://codereview.chromium.org/139803007

git-svn-id: svn://svn.chromium.org/blink/trunk@166662 bbb929c8-8fbe-4397-9dbb-9b2b20218538