Convert most Chromium presubmit files to use new multiple-tryserver-compatible protocol

chrome/browser/ui/libgtk2ui/PRESUBMIT.py

 #!/usr/bin/python
 
 # 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/chrome/browser/ui/libgtk2ui.
 
 See http://dev.chromium.org/developers/how-tos/depottools/presubmit-scripts
 for more details on the presubmit API built into gcl.
 """
 
-def GetPreferredTrySlaves():
-  return ['linux_rel']
+def GetPreferredTryMasters(project, change):
+  return {
+    'tryserver.chromium': {
+      'linux_rel': set(['defaulttests']),
+    }
+  }