Pass mb_mastername and mb_buildername as parameters to override the default.

scripts/slave/recipes/chromium.py

Index: scripts/slave/recipes/chromium.py
diff --git a/scripts/slave/recipes/chromium.py b/scripts/slave/recipes/chromium.py
index ba7fd60448271d1073cf182aca6cb385612ca8c1..8adc0f33b76726a9d62cfcc4ffc935b07dc5909b 100644
--- a/scripts/slave/recipes/chromium.py
+++ b/scripts/slave/recipes/chromium.py
@@ -43,8 +43,8 @@ def RunSteps(api):
         api.json.dumps(api.chromium_tests.builders, default=ignore_undumpable))
     return
 
-  mastername = api.properties.get('mastername')
-  buildername = api.properties.get('buildername')
+  mastername = api.properties['mastername']
+  buildername = api.properties['buildername']
 
   if mastername == 'tryserver.chromium.perf' and api.chromium_tests.builders[
       mastername]['builders'][buildername]['bot_type'] == 'tester':
@@ -54,7 +54,8 @@ def RunSteps(api):
   update_step, master_dict, test_spec = \
       api.chromium_tests.prepare_checkout(mastername, buildername)
   api.chromium_tests.compile(mastername, buildername, update_step, master_dict,
-                             test_spec)
+                             test_spec, mb_mastername=mastername,
+                             mb_buildername=buildername)
   tests = api.chromium_tests.tests_for_builder(
       mastername, buildername, update_step, master_dict)