Update masters after the tryserver split.

net/PRESUBMIT.py

 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """Chromium presubmit script for src/net.
 
 See http://dev.chromium.org/developers/how-tos/depottools/presubmit-scripts
 for more details on the presubmit API built into gcl.
 """
 
 def GetPreferredTryMasters(project, change):
   masters = {
-    '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']),
     }
   }
   # Changes that touch NSS files will likely need a corresponding OpenSSL edit.
   # Conveniently, this one glob also matches _openssl.* changes too.
   if any('nss' in f.LocalPath() for f in change.AffectedFiles()):
-    masters['tryserver.chromium'].setdefault(
+    masters['tryserver.chromium.linux'].setdefault(
       'linux_redux', set()).add('defaulttests')
   return masters