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

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 06c824e1d9f9a62e1493676d5b4d8e7a90b4fedd..a5f1713aa6f06c78a3a52a3186020303a30178b3 100644
--- a/scripts/slave/recipes/chromium.py
+++ b/scripts/slave/recipes/chromium.py
@@ -32,11 +32,15 @@ def GenSteps(api):
   api.chromium_tests.configure_build(mastername, buildername)
   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)
+
   tests = api.chromium_tests.tests_for_builder(
       mastername, buildername, update_step, master_dict)
 
+  api.chromium.run_mb_if_necessary(mastername, buildername, tests)
+
+  api.chromium_tests.compile(mastername, buildername, update_step, master_dict,
+                             test_spec)
+
   if not tests:
     return