Only remove CHROME_SANDBOX_ENV from env if it is present.

testing/test_env.py

 #!/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.
 
 """Sets environment variables needed to run a chromium unit test."""
 
 import os
 import stat
 import subprocess
@@ skipping 30 matching lines @@
     if verbose:
       print 'Enabling sandbox. Setting environment variable:'
       print '  %s="%s"' % (CHROME_SANDBOX_ENV, chrome_sandbox_path)
     env[CHROME_SANDBOX_ENV] = chrome_sandbox_path
   else:
     if verbose:
       print 'Sandbox not properly installed. Unsetting:'
       print '  %s' % CHROME_SANDBOX_ENV
     # The variable should be removed from the environment, making
     # the variable empty silently disables the sandbox.
-    env.pop(CHROME_SANDBOX_ENV)
+    if env.get(CHROME_SANDBOX_ENV):
+      env.pop(CHROME_SANDBOX_ENV)
 
 
 def fix_python_path(cmd):
   """Returns the fixed command line to call the right python executable."""
   out = cmd[:]
   if out[0] == 'python':
     out[0] = sys.executable
   elif out[0].endswith('.py'):
     out.insert(0, sys.executable)
   return out
@@ skipping 21 matching lines @@
     print >> sys.stderr, 'Failed to start %s' % cmd
     raise
 
 
 def main():
   return run_executable(sys.argv[1:], os.environ.copy())
 
 
 if __name__ == '__main__':
   sys.exit(main())