Add support for "additional_mirrors" property in trybot configs.

scripts/slave/recipes/bisection/desktop_bisect.py

Index: scripts/slave/recipes/bisection/desktop_bisect.py
diff --git a/scripts/slave/recipes/bisection/desktop_bisect.py b/scripts/slave/recipes/bisection/desktop_bisect.py
index 5f6210f0c9f63b62a6d06ed627b83a3d4eaf9f8d..44220ce2398db5385e9084b40c2b716e137a29ba 100644
--- a/scripts/slave/recipes/bisection/desktop_bisect.py
+++ b/scripts/slave/recipes/bisection/desktop_bisect.py
@@ -23,10 +23,10 @@ def RunSteps(api):
   # builder.py from chromium_tests recipe module.
   api.chromium_tests.configure_build(mastername, buildername)
   api.gclient.apply_config('perf')
-  update_step, master_dict, _ = \
+  update_step, master_dicts, _ = \
       api.chromium_tests.prepare_checkout(mastername, buildername)
   api.auto_bisect.start_try_job(api, update_step=update_step,
-                                master_dict=master_dict)
+                                master_dict=master_dicts[0])
 
 def GenTests(api):
   yield (api.test('basic')