Gardening: Marking http/tests/media/video-seek-to-middle.html as Flaky on Mac DEbug

BUG=328996
TBR=jchaffraix@chromium.org,dpranke@chromium.org

Review URL: https://codereview.chromium.org/116543006

git-svn-id: svn://svn.chromium.org/blink/trunk@163979 bbb929c8-8fbe-4397-9dbb-9b2b20218538