Mark media/video-frame-size-change.html as flaky on Mac10.6

BUG=326139
TBR=tdanderson

Review URL: https://codereview.chromium.org/138423002

git-svn-id: svn://svn.chromium.org/blink/trunk@165075 bbb929c8-8fbe-4397-9dbb-9b2b20218538