Add support for "additional_mirrors" property in trybot configs.

scripts/slave/recipes/chromium.py

 # Copyright 2013 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.
 
 DEPS = [
   'adb',
   'amp',
   'bisect_tester',
   'bot_update',
   'chromium',
@@ skipping 33 matching lines @@
     return
 
   mastername = api.properties.get('mastername')
   buildername = api.properties.get('buildername')
 
   if mastername == 'tryserver.chromium.perf' and api.chromium_tests.builders[
       mastername]['builders'][buildername]['bot_type'] == 'tester':
     api.bisect_tester.upload_job_url()
 
   api.chromium_tests.configure_build(mastername, buildername)
-  update_step, master_dict, test_spec = \
+  update_step, master_dicts, test_specs = \
       api.chromium_tests.prepare_checkout(mastername, buildername)
-  api.chromium_tests.compile(mastername, buildername, update_step, master_dict,
-                             test_spec)
+  api.chromium_tests.compile(mastername, buildername, update_step, master_dicts,
+                             None, test_specs)
   api.chromium_tests.download_and_unzip_build(mastername, buildername,
-                                              update_step, master_dict)
+                                              update_step, master_dicts)
   tests = api.chromium_tests.tests_for_builder(
-      mastername, buildername, update_step, master_dict)
+      mastername, buildername, update_step, master_dicts[0])

[Sergey Berezin, 2016/01/06 01:50:56]

Is it guaranteed that master_dicts is always of size 1 here? I feel for sanity,
it might make sense to redefine tests_for_builder to take master_dicts. 

 
   if not tests:
     return
 
   api.chromium_tests.configure_swarming('chromium', precommit=False,
                                         mastername=mastername)
   test_runner = api.chromium_tests.create_test_runner(api, tests)
   with api.chromium_tests.wrap_chromium_tests(mastername, tests):
     test_runner()
 
@@ skipping 856 matching lines @@
     api.test('chromium_linux_Android_Tests_logcat_upload_timeout') +
     api.properties(
         mastername='chromium.linux',
         buildername='Android Tests',
         slavename='build1-a1',
         buildnumber='77457',
         parent_buildername='Android Builder',
     ) +
     api.override_step_data('gsutil upload', retcode=-2001)
   )