Suppress coverage information for mac python system libs

This cleans up the output from test-webkitpy -c on the mac a bit.

R=eseidel@chromium.org, ojan@chromium.org

Review URL: https://chromiumcodereview.appspot.com/19721003

git-svn-id: svn://svn.chromium.org/blink/trunk@154474 bbb929c8-8fbe-4397-9dbb-9b2b20218538