Pass mb_mastername and mb_buildername as parameters to override the default.

scripts/slave/recipes/chromium_trybot.py

Index: scripts/slave/recipes/chromium_trybot.py
diff --git a/scripts/slave/recipes/chromium_trybot.py b/scripts/slave/recipes/chromium_trybot.py
index aca5aa1fdabfb98ff06c485268451cd0b72033d7..0cff44c1ff728790a087869f74c056c646260523 100644
--- a/scripts/slave/recipes/chromium_trybot.py
+++ b/scripts/slave/recipes/chromium_trybot.py
@@ -203,17 +203,16 @@ def _RunStepsInternal(api):
     master_dict = api.chromium_tests.trybots.get(mastername, {})
     return master_dict.get('builders', {}).get(buildername)
 
-  mastername = api.properties.get('mastername')
-  buildername = api.properties.get('buildername')
+  mastername = api.properties['mastername']
+  buildername = api.properties['buildername']
   bot_config = get_bot_config(mastername, buildername)
   api.chromium_tests.configure_swarming('chromium', precommit=True)
 
   # TODO(sergiyb): This is a temporary hack to run GPU tests on tryserver
   # only. This should be removed when we will convert chromium.gpu waterfall
   # to swarming and be able to replicate the tests to tryserver automatically.
-  master = api.properties['mastername']
-  builder = api.properties['buildername']
-  enable_gpu_tests = builder in CHROMIUM_GPU_DIMENSION_SETS.get(master, {})
+  enable_gpu_tests = (buildername in
+                      CHROMIUM_GPU_DIMENSION_SETS.get(mastername, {}))
 
   api.chromium_tests.configure_build(
       bot_config['mastername'],
@@ -257,7 +256,8 @@ def _RunStepsInternal(api):
         bot_update_step.presentation.properties['got_revision'],
         bot_update_step.presentation.properties['got_revision'],
         enable_swarming=True,
-        swarming_dimension_sets=CHROMIUM_GPU_DIMENSION_SETS[master][builder]))
+        swarming_dimension_sets=(
+            CHROMIUM_GPU_DIMENSION_SETS[mastername][buildername])))
 
   affected_files = api.tryserver.get_files_affected_by_patch()
 
@@ -308,7 +308,9 @@ def _RunStepsInternal(api):
       api.chromium_tests.analyze(affected_files,
                                  test_targets,
                                  additional_compile_targets,
-                                 'trybot_analyze_config.json')
+                                 'trybot_analyze_config.json',
+                                 mastername,
+                                 buildername)
 
   if bot_config.get('analyze_mode') == 'compile':
     tests = []
@@ -346,6 +348,8 @@ def _RunStepsInternal(api):
         master_dict,
         compile_targets,
         tests_including_triggered,
+        mb_mastername=mastername,
+        mb_buildername=buildername,
         override_bot_type='builder_tester')
   else:
     # Even though the patch doesn't require a compile on this platform,
@@ -361,7 +365,8 @@ def _RunStepsInternal(api):
     return
 
   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)
 
 
 def RunSteps(api):