Add support for "additional_mirrors" property in trybot configs.

scripts/slave/recipes/findit/chromium/test.py

Index: scripts/slave/recipes/findit/chromium/test.py
diff --git a/scripts/slave/recipes/findit/chromium/test.py b/scripts/slave/recipes/findit/chromium/test.py
index ba98cdf2ab4a164e4af67eeee3a3e123ff786008..b95533c90965ecd038aa73752e91ace1b898a433 100644
--- a/scripts/slave/recipes/findit/chromium/test.py
+++ b/scripts/slave/recipes/findit/chromium/test.py
@@ -62,7 +62,7 @@ def _compile_and_test_at_revision(api, target_mastername, target_buildername,
   results = {}
   with api.step.nest('test %s' % str(revision)):
     # Checkout code at the given revision to recompile.
-    bot_update_step, master_dict, _ = \
+    bot_update_step, master_dicts, _ = \
         api.chromium_tests.prepare_checkout(
             target_mastername,
             target_buildername,
@@ -73,7 +73,7 @@ def _compile_and_test_at_revision(api, target_mastername, target_buildername,
         target_mastername,
         target_testername,  # If not tester, this is same as target_buildername.
         bot_update_step,
-        master_dict,
+        master_dicts[0],
         override_bot_type='builder_tester')
 
     tests_to_run = [test for test in all_tests if test.name in requested_tests]
@@ -89,7 +89,7 @@ def _compile_and_test_at_revision(api, target_mastername, target_buildername,
           target_mastername,
           target_buildername,
           bot_update_step,
-          master_dict,
+          master_dicts,
           compile_targets,
           tests_including_triggered=tests_to_run,
           mb_mastername=target_mastername,