Pass mb_mastername and mb_buildername as parameters to override the default.

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..3ad05dd2f1e99e17be1d72b65d65a792f2f3c6d9 100644
--- a/scripts/slave/recipe_modules/perf_try/api.py
+++ b/scripts/slave/recipe_modules/perf_try/api.py
@@ -182,10 +182,12 @@ class PerfTryJobApi(recipe_api.RecipeApi):
       self.m.chromium_tests.transient_check(
           update_step,
           lambda transform_name: self.m.chromium_tests.run_mb_and_compile(
-              compile_targets, None, name_suffix=transform_name('')))
+              compile_targets, None, name_suffix=transform_name(''),
+              mb_mastername=mastername, mb_buildername=buildername))
     else:
       self.m.chromium_tests.run_mb_and_compile(
-          compile_targets, None, name_suffix=' %s' % name)
+          compile_targets, None, name_suffix=' %s' % name,
+          mb_mastername=mastername, mb_buildername=buildername)
 
   def _run_test(self, cfg, **kwargs):
     """Runs test from config and return results."""