Make *some* version of the Mac GN build work again.

R=brettw@chromium.org
TBR=haraken@chromium.org
BUG=431177

Review URL: https://codereview.chromium.org/701043002

git-svn-id: svn://svn.chromium.org/blink/trunk@185222 bbb929c8-8fbe-4397-9dbb-9b2b20218538