Add a variable use_system_xcode to GN.

third_party/WebKit/Source/build/scripts/rule_bison.py

Index: third_party/WebKit/Source/build/scripts/rule_bison.py
diff --git a/third_party/WebKit/Source/build/scripts/rule_bison.py b/third_party/WebKit/Source/build/scripts/rule_bison.py
index 91037072c26667bddb26fefcefe60c1b133855b4..cd99a3747c45bd2f405003a7532bbdfdf018d2b4 100755
--- a/third_party/WebKit/Source/build/scripts/rule_bison.py
+++ b/third_party/WebKit/Source/build/scripts/rule_bison.py
@@ -32,7 +32,7 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-# usage: rule_bison.py INPUT_FILE OUTPUT_DIR [BISON_EXE]
+# usage: rule_bison.py INPUT_FILE OUTPUT_DIR BISON_EXE [DEVELOPER_DIR]
 # INPUT_FILE is a path to either XPathGrammar.y.
 # OUTPUT_DIR is where the bison-generated .cpp and .h files should be placed.
 
@@ -42,13 +42,13 @@ import os.path
 import subprocess
 import sys
 
-assert len(sys.argv) == 3 or len(sys.argv) == 4
+assert len(sys.argv) == 4 or len(sys.argv) == 5
 
 inputFile = sys.argv[1]
 outputDir = sys.argv[2]
-bisonExe = 'bison'
-if len(sys.argv) > 3:
-    bisonExe = sys.argv[3]
+bisonExe = sys.argv[3]
+if len(sys.argv) > 4:
+    os.environ['DEVELOPER_DIR'] = sys.argv[4]
 
 pathToBison = os.path.split(bisonExe)[0]
 if pathToBison: