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

tools/perf/metrics/histogram.py

Index: tools/perf/metrics/histogram.py
diff --git a/tools/perf/metrics/histogram.py b/tools/perf/metrics/histogram.py
index 6a45b1cb3cd29de8a99e9b974dd10ba77d8cf2d4..6711fb179a35093e715b1c6021942fe92d80cb88 100644
--- a/tools/perf/metrics/histogram.py
+++ b/tools/perf/metrics/histogram.py
@@ -25,12 +25,14 @@ def GetHistogramData(histogram_type, histogram_name, tab):
     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.