Fix build/gyp_chromium for supplement.gypi files with no variables.

build/gyp_chromium

 #!/usr/bin/env python
 
 # Copyright (c) 2012 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
@@ skipping 53 matching lines @@
 def GetVarsStringForGN(supplemental_files):
   vars_dict = {}
 
   for supplement in supplemental_files:
     with open(supplement, 'r') as f:
       try:
         file_data = eval(f.read(), {'__builtins__': None}, None)
       except SyntaxError, e:
         e.filename = os.path.abspath(supplement)
         raise
-      variables = file_data.get('variables')
+      variables = file_data.get('variables', [])
       for v in variables:
         vars_dict[v] = '"' + variables[v] + '"'
 
   env_string = os.environ.get('GYP_DEFINES', [])
   items = shlex.split(env_string)
   for item in items:
     tokens = item.split('=', 1)
     if len(tokens) == 2:
       # Escape $ characters which have special meaning to GN.
       vars_dict[tokens[0]] = '"' + tokens[1].replace("$", "\\$") + '"'
@@ skipping 180 matching lines @@
   # to enfore syntax checking.
   syntax_check = os.environ.get('CHROMIUM_GYP_SYNTAX_CHECK')
   if syntax_check and int(syntax_check):
     args.append('--check')
 
   print 'Updating projects from gyp files...'
   sys.stdout.flush()
 
   # Off we go...
   sys.exit(gyp.main(args))