Add closure_compilation to `git cl try` for CLs that touch resources

ui/webui/resources/PRESUBMIT.py

 # Copyright 2016 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import re
 
 
 def PostUploadHook(cl, change, output_api):
   rietveld_obj = cl.RpcServer()
   description = rietveld_obj.get_description(cl.issue)
 
   existing_bots = (change.CQ_INCLUDE_TRYBOTS or '').split(';')
   clean_bots = set(filter(None, map(lambda s: s.strip(), existing_bots)))
   new_bots = clean_bots | set(['tryserver.chromium.linux:closure_compilation'])
   new_tag = 'CQ_INCLUDE_TRYBOTS=%s' % ';'.join(new_bots)
 
   if clean_bots:
     tag_reg = '^CQ_INCLUDE_TRYBOTS=.*$'
     new_description = re.sub(tag_reg, new_tag, description, flags=re.M | re.I)
   else:
     new_description = description + '\n' + new_tag
 
   if new_description == description:
     return []
 
   rietveld_obj.update_description(cl.issue, new_description)
   return [output_api.PresubmitNotifyResult(
       'Automatically added optional Closure bots to run on CQ.')]
+
+
+def GetPreferredTryMasters(project, change):
+  return {
+    'tryserver.chromium.linux': {
+      'closure_compilation': set(['defaulttests']),
+    },
+  }