Break config_default into master_site_configs.

masters/master.chromium.chromiumos/master.cfg

Index: masters/master.chromium.chromiumos/master.cfg
diff --git a/masters/master.chromium.chromiumos/master.cfg b/masters/master.chromium.chromiumos/master.cfg
index 47019da60a1cb1196873599d6a15695cae638db7..740e656cc8c6d99d8b48b0b055f0e8fc2b4419b5 100644
--- a/masters/master.chromium.chromiumos/master.cfg
+++ b/masters/master.chromium.chromiumos/master.cfg
@@ -17,12 +17,10 @@ from master.factory import chromium_factory
 
 # These modules come from scripts/common, which must be in the PYTHONPATH.
 import config
+import master_site_config
 
 
-ActiveMaster = config.Master.ChromiumChromiumOS
-# This is where we sometimes look up things we share with ChromiumOS.
-# The ActiveMaster inherits from Chromium for the rest.
-AlternateMaster = config.Master.ChromiumOS
+ActiveMaster = master_site_config.ChromiumChromiumOS
 
 # Only the production Buildbot master closes the tree on failures.
 TREE_GATE_KEEPER = ActiveMaster.is_production_host
@@ -108,7 +106,7 @@ master_utils.AutoSetupMaster(c, ActiveMaster,
 
 if TREE_GATE_KEEPER:
   import master_gatekeeper_cfg
-  master_gatekeeper_cfg.Update(config, ActiveMaster, AlternateMaster, c)
+  master_gatekeeper_cfg.Update(config, ActiveMaster, c)
 
 ####### PROJECT IDENTITY