Add a variable use_system_xcode to GN.

build/config/mac/compile_xib.py

Index: build/config/mac/compile_xib.py
diff --git a/build/config/mac/compile_xib.py b/build/config/mac/compile_xib.py
index 845f8c179550bbd339cbd306a008671cf8573607..f37ab2d69d5c816ec4858dfdce682b78997674d3 100644
--- a/build/config/mac/compile_xib.py
+++ b/build/config/mac/compile_xib.py
@@ -4,6 +4,7 @@
 
 
 import argparse
+import logging
 import os
 import re
 import subprocess
@@ -18,8 +19,13 @@ def main():
                       help='Path to output bundle.')
   parser.add_argument('-i', '--input', required=True,
                       help='Path to input xib or storyboard.')
+  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
+
   ibtool_args = [
       'xcrun', 'ibtool',
       '--errors', '--warnings', '--notices',