Create classes_util API, change discover to return a list instead of a dict.

tools/telemetry/telemetry/internal/platform/tracing_controller_backend.py

 # Copyright 2014 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 os
 
-from telemetry.core import discover
 from telemetry.core import util
 from telemetry.internal.platform import tracing_agent
 from telemetry.internal.platform.tracing_agent import chrome_tracing_agent
 from telemetry.timeline import trace_data as trace_data_module
 from telemetry.timeline import tracing_category_filter
 from telemetry.timeline import tracing_options
+from telemetry.util import classes_util
 
 
 def _IterAllTracingAgentClasses():
   tracing_agent_dir = os.path.join(
       os.path.dirname(os.path.realpath(__file__)), 'tracing_agent')
-  return discover.DiscoverClasses(
-      tracing_agent_dir, util.GetTelemetryDir(),
-      tracing_agent.TracingAgent).itervalues()
+  return classes_util.DiscoverClasses(
+      tracing_agent_dir, util.GetTelemetryDir(), tracing_agent.TracingAgent)
 
 
 class TracingControllerBackend(object):
   def __init__(self, platform_backend):
     self._platform_backend = platform_backend
     self._current_trace_options = None
     self._current_category_filter = None
     self._current_chrome_tracing_agent = None
     self._supported_agents_classes = [
         agent_classes for agent_classes in _IterAllTracingAgentClasses() if
@@ skipping 37 matching lines @@
     self._current_category_filter = None
     return trace_data_builder.AsData()
 
   def IsChromeTracingSupported(self):
     return chrome_tracing_agent.ChromeTracingAgent.IsSupported(
         self._platform_backend)
 
   @property
   def is_tracing_running(self):
     return self._current_trace_options != None