Use a default string when calling os.getenv in bisect-perf-regression.py.

tools/bisect-perf-regression.py

 #!/usr/bin/env python
 # Copyright (c) 2013 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.
 
 """Performance Test Bisect Tool
 
 This script bisects a series of changelists using binary search. It starts at
 a bad revision where a performance metric has regressed, and asks for a last
 known-good revision. It will then binary search across this revision range by
@@ skipping 698 matching lines @@
 
 
 def SetBuildSystemDefault(build_system, use_goma, goma_dir):
   """Sets up any environment variables needed to build with the specified build
   system.
 
   Args:
     build_system: A string specifying build system. Currently only 'ninja' or
         'make' are supported."""
   if build_system == 'ninja':
-    gyp_var = os.getenv('GYP_GENERATORS')
+    gyp_var = os.getenv('GYP_GENERATORS', default='')
 
     if not gyp_var or not 'ninja' in gyp_var:
       if gyp_var:
         os.environ['GYP_GENERATORS'] = gyp_var + ',ninja'
       else:
         os.environ['GYP_GENERATORS'] = 'ninja'
 
       if IsWindowsHost():
         os.environ['GYP_DEFINES'] = 'component=shared_library '\
             'incremental_chrome_dll=1 disable_nacl=1 fastbuild=1 '\
             'chromium_win_pch=0'
 
   elif build_system == 'make':
     os.environ['GYP_GENERATORS'] = 'make'
   else:
     raise RuntimeError('%s build not supported.' % build_system)
 
   if use_goma:
-    os.environ['GYP_DEFINES'] = '%s %s' % (os.getenv('GYP_DEFINES', ''),
-                                              'use_goma=1')
+    os.environ['GYP_DEFINES'] = '%s %s' % (os.getenv('GYP_DEFINES', default=''),
+                                           'use_goma=1')
     if goma_dir:
       os.environ['GYP_DEFINES'] += ' gomadir=%s' % goma_dir
 
 
 def BuildWithMake(threads, targets, build_type='Release'):
   cmd = ['make', 'BUILDTYPE=%s' % build_type]
 
   if threads:
     cmd.append('-j%d' % threads)
 
@@ skipping 74 matching lines @@
 
       if opts.build_preference == 'msvs':
         if not os.getenv('VS100COMNTOOLS'):
           raise RuntimeError(
               'Path to visual studio could not be determined.')
       else:
         SetBuildSystemDefault(opts.build_preference, opts.use_goma,
                               opts.goma_dir)
     else:
       if not opts.build_preference:
-        if 'ninja' in os.getenv('GYP_GENERATORS'):
+        if 'ninja' in os.getenv('GYP_GENERATORS', default=''):
           opts.build_preference = 'ninja'
         else:
           opts.build_preference = 'make'
 
       SetBuildSystemDefault(opts.build_preference, opts.use_goma, opts.goma_dir)
 
     if not bisect_utils.SetupPlatformBuildEnvironment(opts):
       raise RuntimeError('Failed to set platform environment.')
 
   @staticmethod
@@ skipping 3337 matching lines @@
       # The perf dashboard scrapes the "results" step in order to comment on
       # bugs. If you change this, please update the perf dashboard as well.
       bisect_utils.OutputAnnotationStepStart('Results')
     print 'Error: %s' % e.message
     if opts.output_buildbot_annotations:
       bisect_utils.OutputAnnotationStepClosed()
   return 1
 
 if __name__ == '__main__':
   sys.exit(main())