Correct invalid use of 'is' when '==' should have been used.

subprocess2.py

 # coding=utf8
 # 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.
 """Collection of subprocess wrapper functions.
 
 In theory you shouldn't need anything else in subprocess, or this module failed.
 """
 
 import cStringIO
@@ skipping 277 matching lines @@
       # Pipe but no input, make sure it's closed.
       self.stdin.close()
     for t in threads.itervalues():
       t.start()
 
     timed_out = False
     try:
       # This thread needs to be optimized for speed.
       while threads:
         item = queue.get()
-        if item[0] is 'stdout':
+        if item[0] == 'stdout':
           self.stdout_cb(item[1])
-        elif item[0] is 'stderr':
+        elif item[0] == 'stderr':
           self.stderr_cb(item[1])
         else:
           # A thread terminated.
           threads[item].join()
           del threads[item]
           if item == 'wait':
             # Terminate the timeout thread if necessary.
             done.set()
           elif item == 'timeout' and not timed_out and self.poll() is None:
             logging.debug('Timed out after %fs: killing' % self.timeout)
@@ skipping 129 matching lines @@
 
   - Throws if return code is not 0.
   - Works even prior to python 2.7.
   - Blocks stdin by default if not specified since no output will be visible.
   - As per doc, "The stdout argument is not allowed as it is used internally."
   """
   kwargs.setdefault('stdin', VOID)
   if 'stdout' in kwargs:
     raise ValueError('stdout argument not allowed, it will be overridden.')
   return check_call_out(args, stdout=PIPE, **kwargs)[0]