Making CallStack immutable, so it can be hashable

appengine/findit/crash/project_classifier.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 logging
 
 from crash.occurrence import RankByOccurrence
 from crash.type_enums import CallStackLanguageType
 from model.crash.crash_config import CrashConfig
 
@@ skipping 89 matching lines @@
         return (weight, index)
 
       rank_function = _RankFunctionForJava
 
     top_n_frames = self.project_classifier_config['top_n']
     # If ``results`` are available, we use the projects from there since
     # they're more reliable than the ones from the ``crash_stack``.
     if results:
       classes = map(self.GetClassFromResult, results[:top_n_frames])
     else:
-      classes = map(self.GetClassFromStackFrame, crash_stack[:top_n_frames])
+      classes = map(self.GetClassFromStackFrame,
+          crash_stack.frames[:top_n_frames])
 
     # Since we're only going to return the highest-ranked class, might
     # as well set ``max_classes`` to 1.
     projects = RankByOccurrence(classes, 1, rank_function=rank_function)
 
     if projects:
       return projects[0]
 
     logging.warning('ProjectClassifier.Classify: no projects found.')
     return ''