Add try server configuration for 'git cl try'.

PRESUBMIT.py

Index: PRESUBMIT.py
diff --git a/PRESUBMIT.py b/PRESUBMIT.py
index fe15157dde7f7e237bb85a77725f6e08b89c3714..4f7a9600976a1cdfdfdcbf5b6a3acf152a62ce92 100644
--- a/PRESUBMIT.py
+++ b/PRESUBMIT.py
@@ -98,3 +98,12 @@ def CheckChangeOnCommit(input_api, output_api):
         input_api, output_api,
         json_url='http://v8-status.appspot.com/current?format=json'))
   return results
+
+
+def GetPreferredTryMasters(project, change):
+  return {
+    'tryserver.v8': {
+      'v8_mac_rel': set(['defaulttests']),
+      'v8_win_rel': set(['defaulttests']),
+    },
+  }