sandwich: Refactor to use more existing code.

tools/android/loading/loading_trace.py

 # Copyright (c) 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.
 
 """Represents the trace of a page load."""
 
+import json
 import page_track
 import request_track
 import tracing
 
+
 class LoadingTrace(object):
   """Represents the trace of a page load."""
   _URL_KEY = 'url'
   _METADATA_KEY = 'metadata'
   _PAGE_KEY = 'page_track'
   _REQUEST_KEY = 'request_track'
   _TRACING_KEY = 'tracing_track'
 
   def __init__(self, url, metadata, page, request, tracing_track):
     """Initializes a loading trace instance.
@@ skipping 12 matching lines @@
     self.tracing_track = tracing_track
 
   def ToJsonDict(self):
     """Returns a dictionary representing this instance."""
     result = {self._URL_KEY: self.url, self._METADATA_KEY: self.metadata,
               self._PAGE_KEY: self.page_track.ToJsonDict(),
               self._REQUEST_KEY: self.request_track.ToJsonDict(),
               self._TRACING_KEY: self.tracing_track.ToJsonDict()}
     return result
 
+  def ToJsonFile(self, json_path):
+    """Save a json file representing this instance."""
+    json_dict = self.ToJsonDict()
+    with open(json_path, 'w') as output_file:
+       json.dump(json_dict, output_file, indent=2)
+
   @classmethod
   def FromJsonDict(cls, json_dict):
     """Returns an instance from a dictionary returned by ToJsonDict()."""
     keys = (cls._URL_KEY, cls._METADATA_KEY, cls._PAGE_KEY, cls._REQUEST_KEY,
             cls._TRACING_KEY)
     assert all(key in json_dict for key in keys)
     page = page_track.PageTrack.FromJsonDict(json_dict[cls._PAGE_KEY])
     request = request_track.RequestTrack.FromJsonDict(
         json_dict[cls._REQUEST_KEY])
     tracing_track = tracing.TracingTrack.FromJsonDict(
         json_dict[cls._TRACING_KEY])
     return LoadingTrace(json_dict[cls._URL_KEY], json_dict[cls._METADATA_KEY],
                         page, request, tracing_track)
+
+  @classmethod
+  def FromJsonFile(cls, json_path):
+    """Returns an instance from a json file saved by ToJsonFile()."""
+    with open(json_path) as input_file:
+      return cls.FromJsonDict(json.load(input_file))