[Predator] Make ``CallStackDetector`` return a namedtuple.

appengine/findit/util_scripts/crash_queries/delta_test/delta_test.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 json
 import os
 import pickle
 import subprocess
 
 from crash_queries import crash_iterator
 from crash_queries.delta_test import delta_util
 
 PREDATOR_RESULTS_DIRECTORY = os.path.join(os.path.dirname(__file__),
                                           'predator_results')
 DELTA_TEST_DIRECTORY = os.path.dirname(__file__)
 CRASH_FIELDS = ['crashed_version', 'stack_trace', 'signature',
                 'platform', 'client_id', 'regression_range',
                 'customized_data', 'historical_metadata']
 
 
-
 # TODO(crbug.com/662540): Add unittests.
 class Delta(object):  # pragma: no cover.
   """Stands for delta between two results.
 
   Note, the 2 results should be the same kind and have the same structure.
   """
 
   def __init__(self, result1, result2):
     self._result1 = result1
     self._result2 = result2
@@ skipping 201 matching lines @@
       print '========= Delta of this batch ========='
       delta_util.PrintDelta(batch_deltas, len(crashes), app_id)
       deltas.update(batch_deltas)
 
     return deltas, crash_count
   finally:
     with open(os.devnull, 'w') as null_handle:
       subprocess.check_call(['git', 'checkout', head_branch_name],
                             stdout=null_handle,
                             stderr=null_handle)