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

ui/gl/PRESUBMIT.py

Index: ui/gl/PRESUBMIT.py
diff --git a/ui/gl/PRESUBMIT.py b/ui/gl/PRESUBMIT.py
index 1e529b45f02900529b692cb5f1def7ffacd8d9bc..b1fe6347b84de3d7c9c03b5ccfcccd2457638cb0 100644
--- a/ui/gl/PRESUBMIT.py
+++ b/ui/gl/PRESUBMIT.py
@@ -8,10 +8,12 @@ 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',
-    'mac_gpu',
-    'mac_gpu_retina',
-    'win_gpu',
-  ]
+def GetPreferredTryMasters(project, change):
+  return {
+    'tryserver.chromium': {
+      'linux_gpu': set(['defaulttests']),
+      'mac_gpu': set(['defaulttests']),
+      'mac_gpu_retina': set(['defaulttests']),
+      'win_gpu': set(['defaulttests']),
+    }
+  }