Create a separate function that returns the list of additional...

build/gyp_chromium

 #!/usr/bin/python
 
 # Copyright (c) 2009 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # This script is wrapper for Chromium that adds some support for how GYP
 # is invoked by Chromium beyond what can be done in the gclient hooks.
 
 import glob
 import os
 import shlex
 import sys
 
 script_dir = os.path.dirname(__file__)
 chrome_src = os.path.normpath(os.path.join(script_dir, os.pardir))
 
 sys.path.append(os.path.join(chrome_src, 'tools', 'gyp', 'pylib'))
 import gyp
 
+def additional_include_files(args=[]):
+  """
+  Returns a list of additional (.gypi) files to include, without
+  duplicating ones that are already specified on the command line.
+  """
+  # Determine the include files specified on the command line.
+  # This doesn't cover all the different option formats you can use,
+  # but it's mainly intended to avoid duplicating flags on the automatic
+  # makefile regeneration which only uses this format.
+  specified_includes = set()
+  for arg in args:
+    if arg.startswith('-I') and len(arg) > 2:
+      specified_includes.add(os.path.realpath(arg[2:]))
+
+  result = []
+  def AddInclude(path):
+    if os.path.realpath(path) not in specified_includes:
+      result.append(path)
+
+  # Always include common.gypi
+  AddInclude(os.path.join(script_dir, 'common.gypi'))
+
+  # Optionally add supplemental .gypi files if present.
+  supplements = glob.glob(os.path.join(chrome_src, '*', 'supplement.gypi'))
+  for supplement in supplements:
+    AddInclude(supplement)
+
+  return result
+
 if __name__ == '__main__':
   args = sys.argv[1:]
 
   # This could give false positives since it doesn't actually do real option
   # parsing.  Oh well.
   gyp_file_specified = False
   for arg in args:
     if arg.endswith('.gyp'):
       gyp_file_specified = True
       break
 
   # If we didn't get a file, check an env var, and then fall back to
   # assuming 'all.gyp' from the same directory as the script.
   if not gyp_file_specified:
     gyp_file = os.environ.get('CHROMIUM_GYP_FILE')
     if gyp_file:
       # Note that CHROMIUM_GYP_FILE values can't have backslashes as
       # path separators even on Windows due to the use of shlex.split().
       args.extend(shlex.split(gyp_file))
     else:
       args.append(os.path.join(script_dir, 'all.gyp'))
 
-  # Avoid duplicating an include that's already in the command line.  This
-  # doesn't cover all the different option formats you can use, but it's mainly
-  # intended to avoid duplicating flags on the automatic makefile regeneration
-  # which only uses this format.
-  specified_includes = set()
-  for arg in args:
-    if arg.startswith('-I') and len(arg) > 2:
-      specified_includes.add(os.path.realpath(arg[2:]))
-
-  def AddInclude(path):
-    if os.path.realpath(path) not in specified_includes:
-      args.append('-I' + path)
-
-  # Always include common.gypi
-  AddInclude(os.path.join(script_dir, 'common.gypi'))
-
-  # Optionally add supplemental .gypi files if present.
-  supplements = glob.glob(os.path.join(chrome_src, '*', 'supplement.gypi'))
-  for supplement in supplements:
-    AddInclude(supplement)
+  args.extend(['-I' + i for i in additional_include_files(args)])
 
   print 'Updating projects from gyp files...'
   sys.stdout.flush()
 
   # Off we go...
   sys.exit(gyp.main(args))