Add support for "additional_mirrors" property in trybot configs.

scripts/slave/recipe_modules/perf_try/api.py

Index: scripts/slave/recipe_modules/perf_try/api.py
diff --git a/scripts/slave/recipe_modules/perf_try/api.py b/scripts/slave/recipe_modules/perf_try/api.py
index 8afc07941938b5b06b01fb8ccd1c1b705b92ffba..6d7a07e705d8472af671f73ce6de02c3f0ac292f 100644
--- a/scripts/slave/recipe_modules/perf_try/api.py
+++ b/scripts/slave/recipe_modules/perf_try/api.py
@@ -160,15 +160,16 @@ class PerfTryJobApi(recipe_api.RecipeApi):
     return update_step
 
   def _compile(self, name, mastername, buildername, update_step,
-               master_dict, test_spec=None):
+               master_dict, test_specs=None):
     """Runs compile and related steps for given builder."""
-    if test_spec is None:
-      test_spec = {}
+    if test_specs is None:
+      test_specs = [{}]
     compile_targets, _ = self.m.chromium_tests.get_compile_targets_and_tests(
         mastername,
         buildername,
-        master_dict,
-        test_spec,
+        [master_dict],
+        None,
+        test_specs,
         override_bot_type='builder_tester',
         override_tests=[])
     if self.m.chromium.c.TARGET_PLATFORM == 'android':