Update masters after the tryserver split.

base/PRESUBMIT.py

Index: base/PRESUBMIT.py
diff --git a/base/PRESUBMIT.py b/base/PRESUBMIT.py
index 1a1cce75def4c27183e122791928aba5fc2422b0..931f94ae954181b05848ee8a91407f52e4addff4 100644
--- a/base/PRESUBMIT.py
+++ b/base/PRESUBMIT.py
@@ -50,9 +50,13 @@ def CheckChangeOnCommit(input_api, output_api):
 
 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']),
     }
   }