Refactor stages 1, 2 and test_api overhaul.

scripts/slave/recipe_modules/chromium/api.py

Index: scripts/slave/recipe_modules/chromium/api.py
diff --git a/scripts/slave/recipe_modules/chromium/api.py b/scripts/slave/recipe_modules/chromium/api.py
index 4148bab75d75e41fb2ae18dbe28f0712c2963cef..870172440c71bc3accdd00184853fb487c1b9204 100644
--- a/scripts/slave/recipe_modules/chromium/api.py
+++ b/scripts/slave/recipe_modules/chromium/api.py
@@ -322,7 +322,7 @@ class ChromiumApi(recipe_api.RecipeApi):
       full_args.append('--test-type=%s' % test_type)
     step_name = name or t_name
     full_args.append('--step-name=%s' % step_name)
-    if chartjson_file:
+    if chartjson_file and not 'step_test_data' in kwargs:
       full_args.append('--chartjson-file')
       full_args.append(self.m.json.output())
       kwargs['step_test_data'] = lambda: self.m.json.test_api.output([])