Add swarming support for MB (and the GN bots) in the chromium recipes.

scripts/slave/recipe_modules/chromium_tests/api.py

Index: scripts/slave/recipe_modules/chromium_tests/api.py
diff --git a/scripts/slave/recipe_modules/chromium_tests/api.py b/scripts/slave/recipe_modules/chromium_tests/api.py
index 25c29b51bd345aff9a20e1c83fc2a6ba1fb75942..c1a2d64fd3098b2ae73de7f1b26ebe2eeec7899c 100644
--- a/scripts/slave/recipe_modules/chromium_tests/api.py
+++ b/scripts/slave/recipe_modules/chromium_tests/api.py
@@ -127,9 +127,6 @@ class ChromiumTestsApi(recipe_api.RecipeApi):
     test_spec_file = bot_config.get('testing', {}).get('test_spec_file',
                                                        '%s.json' % mastername)
 
-    if self.m.chromium.c.project_generator.tool == 'mb':
-      self.m.chromium.run_mb(mastername, buildername)
-
     # TODO(phajdan.jr): Bots should have no generators instead.
     if bot_config.get('disable_tests'):
       test_spec = {}