Testexpectations: Add back a line for a flaky test

This line was commented out in http://crrev.com/122463005
And removed (incorrectly) by auto-rebaseline bot in:
http://crrev.com/134313003

BUG=331619
R=jzern@chromium.org

Review URL: https://codereview.chromium.org/137403003

git-svn-id: svn://svn.chromium.org/blink/trunk@165010 bbb929c8-8fbe-4397-9dbb-9b2b20218538