Fix overflow for 32-bit jiffie counter on Android.

tools/perf/metrics/cpu_unittest.py

Index: tools/perf/metrics/cpu_unittest.py
diff --git a/tools/perf/metrics/cpu_unittest.py b/tools/perf/metrics/cpu_unittest.py
index 30d488dac8bc420399e00f32ea8a6fff6b8277d0..6c7f893d0a18a6582a129bc1f18d4072ca3ca1f7 100644
--- a/tools/perf/metrics/cpu_unittest.py
+++ b/tools/perf/metrics/cpu_unittest.py
@@ -31,3 +31,9 @@ class CpuMetricTest(unittest.TestCase):
     self.assertEqual({'Browser': 0.25, 'Renderer': 0.1},
                      cpu._SubtractCpuStats(end, start))
 
+    # Test 32-bit overflow.
+    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)
+