Mark activation-behavior.window.html layout test as flaky on Mac

BUG=824848
TBR=dom-dev@chromium.org,qyearsley@chromium.org

Change-Id: I9b80a22d0084a151fb13e41ebb9bf2f409160cc4
Reviewed-on: https://chromium-review.googlesource.com/c/1461683
Reviewed-by: Karan Bhatia <karandeepb@chromium.org>
Reviewed-by: Quinten Yearsley <qyearsley@chromium.org>
Commit-Queue: Karan Bhatia <karandeepb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#630613}
1 file changed