Add support for "additional_mirrors" property in trybot configs.

scripts/slave/recipe_modules/auto_bisect/api.py

Index: scripts/slave/recipe_modules/auto_bisect/api.py
diff --git a/scripts/slave/recipe_modules/auto_bisect/api.py b/scripts/slave/recipe_modules/auto_bisect/api.py
index a461a017c9c9889c419342176af9cf3c90767dcc..fd345e3502155968577f213bb5fd4e1b446edb4f 100644
--- a/scripts/slave/recipe_modules/auto_bisect/api.py
+++ b/scripts/slave/recipe_modules/auto_bisect/api.py
@@ -131,11 +131,11 @@ class AutoBisectApi(recipe_api.RecipeApi):
     buildername = api.properties.get('buildername')
     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,
             root_solution_revision=test_config_params['revision'])
-    self.start_test_run_for_bisect(api, update_step, master_dict,
+    self.start_test_run_for_bisect(api, update_step, master_dicts[0],

[luqui, 2016/01/05 23:02:35]

This [0] is suspicious.  Is it clear during construction that the first element
is distinguished in this way (or does it not matter for some reason)?

[Ken Russell (switch to Gerrit), 2016/01/06 00:38:24]

No, I didn't do a good job documenting the return values from
api.chromium_tests.prepare_checkout. I agree this approach is pretty hacky.
Basically, I found it necessary to return multiple master_dicts and test_specs
from prepare_checkout, so both return values are arrays now. The first entry in
each array is what would previously have been returned in master_dict and
test_spec. Subsequent entries correspond to the "additional_mirrors" entry in
trybots.py.

I may have missed something major in the design here, but it does seem to me
that it's necessary to return both multiple master_dict and test_spec instances.
However, api.chromium_tests.get_master_dict_with_dynamic_tests does some work
which makes me wonder if all the data in test_spec is actually embedded already
in the returned master_dict instance.

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

I suspect that bisect would only run waterfall builders, which would only have
one entry in master_dicts. I feel it'd be cleaner to modify
start_test_run_for_bisect() to take master_dicts, but that would probably
trigger too much other code change... 

I'm actually not sure what would be a simple good solution here.

                                    test_config_params, run_locally=True)
 
   def start_test_run_for_bisect(self, api, update_step, master_dict,