Change RunPythonUnitTests() to run the unit tests in a separate process.

presubmit_support.py

 #!/usr/bin/python
 # Copyright (c) 2006-2009 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.
 
 """Enables directory-specific presubmit checks to run at upload and/or commit.
 """
 
 __version__ = '1.3.2'
 
@@ skipping 179 matching lines @@
     self.os_path = os.path
     self.pickle = pickle
     self.marshal = marshal
     self.re = re
     self.subprocess = subprocess
     self.tempfile = tempfile
     self.traceback = traceback
     self.unittest = unittest
     self.urllib2 = urllib2
 
+    # To easily fork python.
+    self.python_executable = sys.executable
+    self.environ = os.environ
+
     # InputApi.platform is the platform you're currently running on.
     self.platform = sys.platform
 
     # The local path of the currently-being-processed presubmit script.
     self._current_presubmit_path = os.path.dirname(presubmit_path)
 
     # We carry the canned checks so presubmit scripts can easily use them.
     self.canned_checks = presubmit_canned_checks
 
   def PresubmitLocalPath(self):
@@ skipping 694 matching lines @@
                                options.commit,
                                options.verbose,
                                sys.stdout,
                                sys.stdin,
                                options.default_presubmit,
                                options.may_prompt)
 
 
 if __name__ == '__main__':
   sys.exit(Main(sys.argv))