Fix overflow for 32-bit jiffie counter on Android.

tools/perf/metrics/cpu_unittest.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.
 
 import unittest
 
 from metrics import cpu
 
 # Testing private method.
 # pylint: disable=W0212
@@ skipping 13 matching lines @@
 
     # A process type will be ignored if there's an empty dict for start or end.
     start['Renderer'] = {}
     self.assertEqual({'Browser': 0.25}, cpu._SubtractCpuStats(end, start))
 
     # Results for multiple process types can be computed.
     start['Renderer'] = {'CpuProcessTime': 0, 'TotalTime': 0}
     self.assertEqual({'Browser': 0.25, 'Renderer': 0.1},
                      cpu._SubtractCpuStats(end, start))
 
+    # Test 32-bit overflow

[Sami, 2014/06/30 16:25:16]

nit: Add '.' at end.

+    start = {'Browser': {'CpuProcessTime': 0, 'TotalTime': 2**32 - 20}}
+    end = {'Browser': {'CpuProcessTime': 5, 'TotalTime': 20}}
+    self.assertEqual({'Browser': 0.125}, cpu._SubtractCpuStats(end, start))
+    self.assertRaises(AssertionError, cpu._SubtractCpuStats, start, end)
+