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

skia/PRESUBMIT.py

 # Copyright 2013 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.
 
 """Top-level presubmit script for skia/
 
 See http://dev.chromium.org/developers/how-tos/depottools/presubmit-scripts
 for more details about the presubmit API built into depot_tools.
 """
 
-def GetPreferredTrySlaves(project, change):
-  return [
-    'linux_gpu',
-    'win_gpu',
-    'mac_gpu',
-    'mac_gpu_retina',
-    'linux_layout_rel',
-    ]
+def GetPreferredTryMasters(project, change):
+  return {
+    'tryserver.chromium': {
+      'linux_gpu': set(['defaulttests']),
+      'win_gpu': set(['defaulttests']),
+      'mac_gpu': set(['defaulttests']),
+      'mac_gpu_retina': set(['defaulttests']),
+      'linux_layout_rel': set(['defaulttests']),
+    }
+  }