[telemetry] Add TraceResult as return value from tracing, and placeholder trace_event_importer.

tools/telemetry/telemetry/run_tests.py

 # Copyright (c) 2012 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 fnmatch
 import logging
 import os
 import traceback
 import unittest
 
 from telemetry import browser_options
 from telemetry import options_for_unittests
 
 def RequiresBrowserOfType(*types):
   def wrap(func):
     func._requires_browser_types = types
     return func
   return wrap
 
 def Discover(start_dir, pattern = 'test*.py', top_level_dir = None):
-  if hasattr(unittest.defaultTestLoader, 'discover'):
-    return unittest.defaultTestLoader.discover( # pylint: disable=E1101
-      start_dir,
-      pattern,
-      top_level_dir)
-
   modules = []
   for dirpath, _, filenames in os.walk(start_dir):
     for filename in filenames:
       if not filename.endswith('.py'):
         continue
 
       if not fnmatch.fnmatch(filename, pattern):
         continue
 
       if filename.startswith('.') or filename.startswith('_'):
@@ skipping 98 matching lines @@
   try:
     os.chdir(top_level_dir)
     for _ in range(
         default_options.run_test_repeat_count): # pylint: disable=E1101
       num_errors += DiscoverAndRunTests(start_dir, args, top_level_dir)
   finally:
     os.chdir(olddir)
     options_for_unittests.Set(None, None)
 
   return min(num_errors, 255)