Mark fast/transforms/shadows.html as flaky on Snow Leopard

TBR=rjwright@chromium.org
BUG=400978

Review URL: https://codereview.chromium.org/450483002

git-svn-id: svn://svn.chromium.org/blink/trunk@179638 bbb929c8-8fbe-4397-9dbb-9b2b20218538