Pass mb_mastername and mb_buildername as parameters to override the default.

scripts/slave/recipes/chromium_gn.py

Index: scripts/slave/recipes/chromium_gn.py
diff --git a/scripts/slave/recipes/chromium_gn.py b/scripts/slave/recipes/chromium_gn.py
index f5f9046052e555d3df1167c0d77368b721163741..b73fd2b915199cd9a4db888ada328da8b4176424 100644
--- a/scripts/slave/recipes/chromium_gn.py
+++ b/scripts/slave/recipes/chromium_gn.py
@@ -188,8 +188,8 @@ def all_compile_targets(api, tests):
 
 
 def _RunStepsInternal(api):
-  mastername = api.properties.get('mastername')
-  buildername = api.properties.get('buildername')
+  mastername = api.properties['mastername']
+  buildername = api.properties['buildername']
   bot_config = BUILDERS[mastername]['builders'][buildername]
   is_android = ('Android' in buildername or 'android' in buildername)
   force_clobber = bot_config.get('force_clobber', False)
@@ -231,7 +231,9 @@ def _RunStepsInternal(api):
             affected_files,
             test_targets,
             additional_compile_targets,
-            'trybot_analyze_config.json')
+            'trybot_analyze_config.json',
+            mastername,
+            buildername)
     if compile_targets:
       api.chromium.run_mb(mastername, buildername, use_goma=True)
       api.chromium.compile(compile_targets,
@@ -246,7 +248,8 @@ def _RunStepsInternal(api):
   if tests:
     if api.tryserver.is_tryserver:
       api.chromium_tests.run_tests_on_tryserver(
-          mastername, api, tests, bot_update_step, affected_files)
+          mastername, api, tests, bot_update_step, affected_files,
+          mb_mastername=mastername, mb_buildername=buildername)
     else:
       api.chromium_tests.configure_swarming('chromium', precommit=False,
                                             mastername=mastername)