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

skia/PRESUBMIT.py

Index: skia/PRESUBMIT.py
diff --git a/skia/PRESUBMIT.py b/skia/PRESUBMIT.py
index e22733cf670cc3978e85e5a17bf9233c43dd9cd6..8da3859c2c6bc7710d6d1cda5e3e22868df61f06 100644
--- a/skia/PRESUBMIT.py
+++ b/skia/PRESUBMIT.py
@@ -8,11 +8,13 @@ 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']),
+    }
+  }