Move memory-related histogram data collection to metrics/memory.py

tools/perf/metrics/histogram.py

 # Copyright 2013 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.
 
 """This is a helper module to get and manipulate histogram data.
 
 The histogram data is the same data as is visible from "chrome://histograms".
 More information can be found at: chromium/src/base/metrics/histogram.h
 
 Histogram data is collected with either the window.statsCollectionController
 object or the window.domAutomationController object.
 """
 
 import json
 import logging
 
 BROWSER_HISTOGRAM = 'browser_histogram'
 RENDERER_HISTOGRAM = 'renderer_histogram'
 
 def GetHistogramData(histogram_type, histogram_name, tab):
   """Get a json serialization of a histogram."""
   assert histogram_type in [BROWSER_HISTOGRAM, RENDERER_HISTOGRAM]
   function = 'getHistogram'
   if histogram_type == BROWSER_HISTOGRAM:
     function = 'getBrowserHistogram'
   # TODO(jeremy): Remove references to
   # domAutomationController when we update the reference builds.
-  return tab.EvaluateJavaScript(
+  histogram_json = tab.EvaluateJavaScript(
       '(window.statsCollectionController ? '
       'statsCollectionController : '
       'domAutomationController).%s("%s")' %
       (function, histogram_name))
-
+  if histogram_json:
+    return histogram_json
+  return None
 
 def SubtractHistogram(histogram_json, start_histogram_json):
   """Subtracts a previous histogram from a histogram.
 
   Both parameters and the returned result are json serializations.
   """
   start_histogram = json.loads(start_histogram_json)
   # It's ok if the start histogram is empty (we had no data, maybe even no
   # histogram at all, at the start of the test).
   if 'buckets' not in start_histogram:
@@ skipping 19 matching lines @@
       if new_bucket['count'] < 0:
         logging.error('Histogram subtraction error, starting histogram most '
                       'probably invalid.')
     if new_bucket['count']:
       new_buckets.append(new_bucket)
   histogram['buckets'] = new_buckets
   histogram['count'] -= start_histogram['count']
 
   return json.dumps(histogram)