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

build/gyp_chromium

Index: build/gyp_chromium
diff --git a/build/gyp_chromium b/build/gyp_chromium
index 1fa030fdd16517c1123a953d1d2710ab5049cfbe..b165f70530b44c39d7698b32199a16a5602c5ba7 100755
--- a/build/gyp_chromium
+++ b/build/gyp_chromium
@@ -145,7 +145,7 @@ if __name__ == '__main__':
   if sys.platform.startswith('linux') and not os.environ.get('GYP_GENERATORS'):
     os.environ['GYP_GENERATORS'] = 'ninja'
   elif sys.platform == 'darwin' and not os.environ.get('GYP_GENERATORS') and \
-      not 'OS=ios' in os.environ.get('GYP_DEFINES'):
+      not 'OS=ios' in os.environ.get('GYP_DEFINES', []):
     os.environ['GYP_GENERATORS'] = 'ninja'
 
   # If CHROMIUM_GYP_SYNTAX_CHECK is set to 1, it will invoke gyp with --check