Fix broken build on OSX >= 10.7.

The breakage appears to be introduced by this CL.
https://codereview.chromium.org/120773007

TEST=ninja -C out/Debug blink_tests
BUG=331719
NOTRY=true

Review URL: https://codereview.chromium.org/124863002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243091 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243091