BuildBucket recipe module can expose properties.

scripts/slave/recipe_modules/chromite/api.py

Index: scripts/slave/recipe_modules/chromite/api.py
diff --git a/scripts/slave/recipe_modules/chromite/api.py b/scripts/slave/recipe_modules/chromite/api.py
index 43482d2049bf61d51dea4f5c14a6e4f96d4fc6bf..db25c1ec2978ffe01d68b472a5d33b869e7f290c 100644
--- a/scripts/slave/recipe_modules/chromite/api.py
+++ b/scripts/slave/recipe_modules/chromite/api.py
@@ -70,9 +70,9 @@ class ChromiteApi(recipe_api.RecipeApi):
     if 'buildnumber' in self.m.properties:
       defaults['CBB_BUILD_NUMBER'] = int(self.m.properties['buildnumber'])
 
-    if 'buildbucket' in self.m.properties:
-      buildbucket_json = self.m.json.loads(self.m.properties.get('buildbucket'))
-      defaults['CBB_BUILDBUCKET_ID'] = buildbucket_json['build']['id']
+    buildbucket_props = self.m.buildbucket.properties
+    if buildbucket_props:
+      defaults['CBB_BUILDBUCKET_ID'] = buildbucket_props['build']['id']
 
     return defaults