[Findit] Add iterator and crash_iterator for delta test

appengine/findit/model/crash/chrome_crash_analysis.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.
 
 from google.appengine.ext import ndb
 
 from model.crash.crash_analysis import CrashAnalysis
 
 
 class ChromeCrashAnalysis(CrashAnalysis):
   """Represents an analysis of a Chrome Crash (Cracas or Fracas)."""
   # Customized properties for Fracas crash.
   historical_metadata = ndb.JsonProperty(indexed=False)
   channel = ndb.StringProperty(indexed=False)
 
   def Reset(self):
     super(ChromeCrashAnalysis, self).Reset()
     self.historical_metadata = None
     self.channel = None
+
+  @property
+  def customized_data(self):
+    return {'historical_metadata': self.historical_metadata,
+            'channel': self.channel}