mac: Don't crash in build/gyp_chromium if GYP_DEFINES isn't set.

BUG=294387
TBR=mark

Review URL: https://codereview.chromium.org/51113011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232015 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 232015