Layout Test video-poster-cross-origin-crash.html is flaky on Mac 10.6

Unreviewed gardening.

BUG=310292
TBR=dpranke@chromium.org

Review URL: https://codereview.chromium.org/32193005

git-svn-id: svn://svn.chromium.org/blink/trunk@160253 bbb929c8-8fbe-4397-9dbb-9b2b20218538