Enable some profile_chrome unit tests on Trybots

systrace/profile_chrome/ddms_tracing_agent_unittest.py

 # Copyright 2015 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.
 
 from profile_chrome import agents_unittest
 from profile_chrome import ddms_tracing_agent
+from systrace import decorators
 
 
 class DdmsAgentTest(agents_unittest.BaseAgentTest):
+  @decorators.NeverRunTest
   def testTracing(self):
     agent = ddms_tracing_agent.DdmsAgent(self.device, self.package_info)
 
     try:
       agent.StartAgentTracing(None)
     finally:
       agent.StopAgentTracing()
 
     result = agent.GetResults()
     self.assertTrue(result.raw_data.startswith('*version'))