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

gpu/PRESUBMIT.py

Index: gpu/PRESUBMIT.py
diff --git a/gpu/PRESUBMIT.py b/gpu/PRESUBMIT.py
index fbacccd617ac4dcabc2828d49c833a6300a93250..a1d7ecff16d8ca7e7c7dc61c448135cb449f5063 100644
--- a/gpu/PRESUBMIT.py
+++ b/gpu/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']),
+    }
+  }