Add BuildBucket manifest scheduling support.

scripts/slave/recipe_modules/chromite/config.py

Index: scripts/slave/recipe_modules/chromite/config.py
diff --git a/scripts/slave/recipe_modules/chromite/config.py b/scripts/slave/recipe_modules/chromite/config.py
index 529bbc367f71e4272213aed16e17642990592a4c..66fae60692b8f58446c4e03431c28acd33e9e4a9 100644
--- a/scripts/slave/recipe_modules/chromite/config.py
+++ b/scripts/slave/recipe_modules/chromite/config.py
@@ -11,7 +11,7 @@ from recipe_engine.config import Dict, Single, Set
 
 def BaseConfig(CBB_CONFIG=None, CBB_BRANCH=None, CBB_BUILD_NUMBER=None,
                CBB_DEBUG=False, CBB_CLOBBER=False, CBB_BUILDBUCKET_ID=None,
-               **_kwargs):
+               CBB_MASTER_BUILD_ID=None, **_kwargs):
   return ConfigGroup(
     # Base mapping of repository key to repository name.
     repositories = Dict(value_type=Set(basestring)),
@@ -47,7 +47,7 @@ def BaseConfig(CBB_CONFIG=None, CBB_BRANCH=None, CBB_BUILD_NUMBER=None,
       builddir = Single(basestring),
 
       # If supplied, forward to cbuildbot as '--master-build-id'.
-      build_id = Single(basestring),
+      build_id = Single(basestring, empty_val=CBB_MASTER_BUILD_ID),
 
       # If supplied, forward to cbuildbot as '--buildnumber'.
       build_number = Single(int, empty_val=CBB_BUILD_NUMBER),