Telemetry / Memory benchmark fix: Separate histograms for different tests.

tools/perf/perf_tools/histogram_unittest.py

+# Copyright (c) 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.
+import json
+import unittest
+
+from perf_tools import histogram as histogram_module
+
+class TestHistogram(unittest.TestCase):
+  def testSubtractHistogram(self):
+    baseline_histogram = """{"count": 3, "buckets": [
+{"low": 1, "high": 2, "count": 1},
+{"low": 2, "high": 3, "count": 2}]}"""
+
+    histogram = """{"count": 14, "buckets": [
+{"low": 1, "high": 2, "count": 1},
+{"low": 2, "high": 3, "count": 3},
+{"low": 3, "high": 4, "count": 10}]}"""
+
+    new_histogram = json.loads(
+        histogram_module.SubtractHistogram(histogram, baseline_histogram))
+    new_buckets = dict()
+    for b in new_histogram['buckets']:
+      new_buckets[b['low']] = b['count']
+    self.assertFalse(1 in new_buckets)
+    self.assertEquals(1, new_buckets[2])
+    self.assertEquals(10, new_buckets[3])