Update masters after the tryserver split.

sync/PRESUBMIT.py

Index: sync/PRESUBMIT.py
diff --git a/sync/PRESUBMIT.py b/sync/PRESUBMIT.py
index f82e95a5435382c1f912ecfe847d6b7927b7cc62..53c8d45e7959899a7cf4eb493a45fa3ed3ba06bc 100644
--- a/sync/PRESUBMIT.py
+++ b/sync/PRESUBMIT.py
@@ -10,9 +10,13 @@ for more details on the presubmit API built into gcl.
 
 def GetPreferredTryMasters(project, change):
   return {
-    'tryserver.chromium': {
-      'linux_chromium_rel': set(['defaulttests']),
+    'tryserver.chromium.linux': {
+      'linux_chromium_rel_swarming': set(['defaulttests']),
+    },
+    'tryserver.chromium.mac': {
       'mac_chromium_rel': set(['defaulttests']),
+    },
+    'tryserver.chromium.win': {
       'win_chromium_rel': set(['defaulttests']),
     }
   }