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

chrome/browser/policy/PRESUBMIT.py

Index: chrome/browser/policy/PRESUBMIT.py
diff --git a/chrome/browser/policy/PRESUBMIT.py b/chrome/browser/policy/PRESUBMIT.py
index da65b741c2a58647e831e6a37ac7a0f314a1baf3..b3c43e5a7d83eb1945c067e70abec6dcf3c19440 100644
--- a/chrome/browser/policy/PRESUBMIT.py
+++ b/chrome/browser/policy/PRESUBMIT.py
@@ -8,8 +8,10 @@ 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_chromeos',
-    'linux_chromeos_clang:compile',
-  ]
+def GetPreferredTryMasters(project, change):
+  return {
+    'tryserver.chromium': {
+      'linux_chromeos': set(['defaulttests']),
+      'linux_chromeos_clang': set(['compile']),
+    }
+  }