Mark svg/animations/repeatn-event-1{b,d}.svg as flaky

TBR=tdanderson
BUG=333791

Review URL: https://codereview.chromium.org/136393002

git-svn-id: svn://svn.chromium.org/blink/trunk@164957 bbb929c8-8fbe-4397-9dbb-9b2b20218538