Add a virtualenv-based python bootstrapping service to infra.

PRESUBMIT.py

 # Copyright (c) 2014 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.
 
 """Top-level presubmit script for buildbot.
 
 See http://dev.chromium.org/developers/how-tos/depottools/presubmit-scripts for
 details on the presubmit API built into gcl.
 """
 
 
 DISABLED_TESTS = [
     '.*appengine/chromium_status/tests/main_test.py',
     '.*appengine/chromium_build/app_test.py',
 ]
 
 
 DISABLED_PROJECTS = [
     'appengine/chromium_build',
     'infra/services/lkgr_finder',
     'infra/services/gnumbd',
-    'infra/ext/[^/]*/.*',
 ]
 
 
 def CommonChecks(input_api, output_api):
   tests = []
 
   blacklist = list(input_api.DEFAULT_BLACK_LIST) + DISABLED_PROJECTS
 
   status_output = input_api.subprocess.check_output(
       ['git', 'status', '--porcelain', '--ignored'])
@@ skipping 29 matching lines @@
 
 
 def CheckChangeOnUpload(input_api, output_api):
   return CommonChecks(input_api, output_api)
 
 
 def CheckChangeOnCommit(input_api, output_api):
   output = CommonChecks(input_api, output_api)
   output.extend(input_api.canned_checks.CheckOwners(input_api, output_api))
   return output