Add simplejson 2.1.0 and use it when it's not already installed.

presubmit_support.py

Index: presubmit_support.py
diff --git a/presubmit_support.py b/presubmit_support.py
index d1c54f52cdb66675e954f562021f434fd7d023f2..cfdcfa38a83d67cd652f27caa7cad1d90710de32 100755
--- a/presubmit_support.py
+++ b/presubmit_support.py
@@ -34,14 +34,14 @@ import unittest  # Exposed through the API.
 import urllib2  # Exposed through the API.
 import warnings
 
-# json may not be available.
 try:
   import simplejson as json
 except ImportError:
   try:
     import json
   except ImportError:
-    json = None
+    # Import the one included in depot_tools.
+    import third_party.simplejson as json
 
 # Local imports.
 import gcl