mac: Fix build with 10.7+ sdk.

BUG=306348
TBR=rsesek@chromium.org

Review URL: https://codereview.chromium.org/145893002

git-svn-id: svn://svn.chromium.org/blink/trunk@165650 bbb929c8-8fbe-4397-9dbb-9b2b20218538