Update masters after the tryserver split.

chrome/common/net/PRESUBMIT.py

Index: chrome/common/net/PRESUBMIT.py
diff --git a/chrome/common/net/PRESUBMIT.py b/chrome/common/net/PRESUBMIT.py
index 61a8cf2859e318ece8bb811f9f626ae3a98c5e1f..f5ec5c1dac360e7add6d924ac5b7c98e5b900af2 100644
--- a/chrome/common/net/PRESUBMIT.py
+++ b/chrome/common/net/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']),
     }
   }