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

ash/PRESUBMIT.py

Index: ash/PRESUBMIT.py
diff --git a/ash/PRESUBMIT.py b/ash/PRESUBMIT.py
index 12b0c42e854f0936c30c7f75764c584f8acaf3cb..648c6639fa9d5dd6026d46742fa62ecb64ae69ed 100644
--- a/ash/PRESUBMIT.py
+++ b/ash/PRESUBMIT.py
@@ -8,5 +8,9 @@ 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_clang']
+def GetPreferredTryMasters(project, change):
+  return {
+    'tryserver.chromium': {
+      'linux_chromeos_clang': set(['defaulttests']),
+    }
+  }