Add a variable use_system_xcode to GN.

build/config/mac/xcrun.py

Index: build/config/mac/xcrun.py
diff --git a/build/config/mac/xcrun.py b/build/config/mac/xcrun.py
index e2a775e51588bc58aea832544c35e2ef18cd5061..bdf15ffffc14f069b63acfefc00931942a699163 100644
--- a/build/config/mac/xcrun.py
+++ b/build/config/mac/xcrun.py
@@ -12,8 +12,13 @@ if __name__ == '__main__':
       description='A script to execute a command via xcrun.')
   parser.add_argument('--stamp', action='store', type=str,
       help='Write a stamp file to this path on success.')
+  parser.add_argument('--developer_dir', required=False,
+                      help='Path to Xcode.')
   args, unknown_args = parser.parse_known_args()
 
+  if (args.developer_dir):
+    os.environ['DEVELOPER_DIR'] = args.developer_dir
+
   rv = subprocess.check_call(['xcrun'] + unknown_args)
   if rv == 0 and args.stamp:
     if os.path.exists(args.stamp):