Mark virtual/gpu/compositedscrolling/overflow/invisible-descendants-should-not-affect-opt-in.html as flaky

Flakiness dashboard is here:
http://test-results.appspot.com/dashboards/flakiness_dashboard.html#group=%40ToT%20Blink&tests=virtual/gpu/compositedscrolling/overflow/invisible-descendants-should-not-affect-opt-in.html

BUG=346134

Review URL: https://codereview.chromium.org/176973002

git-svn-id: svn://svn.chromium.org/blink/trunk@167661 bbb929c8-8fbe-4397-9dbb-9b2b20218538