[telemetry] Fix CPU metric jiffie overflow correction.

tools/perf/metrics/cpu.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 logging
 
 from telemetry.value import scalar
 
 from metrics import Metric
 
@@ skipping 69 matching lines @@
     assert process_type in start_cpu_stats, 'Mismatching process types'
     # Skip any process_types that are empty.
     if (not cpu_stats[process_type]) or (not start_cpu_stats[process_type]):
       continue
     cpu_process_time = (cpu_stats[process_type]['CpuProcessTime'] -
                         start_cpu_stats[process_type]['CpuProcessTime'])
     total_time = (cpu_stats[process_type]['TotalTime'] -
                   start_cpu_stats[process_type]['TotalTime'])
     # Fix overflow for 32-bit jiffie counter, 64-bit counter will not overflow.
     # Linux kernel starts with a value close to an overflow, so correction is
-    # necessary.
+    # necessary. Assume jiffie counter is at 100 Hz.
     if total_time < 0:
-      total_time += 2 ** 32
+      total_time += 2 ** 32 / 100.
     # Assert that the arguments were given in the correct order.
-    assert total_time > 0 and total_time < 2 ** 31, (
+    assert total_time > 0 and total_time < 2 ** 31 / 100., (
         'Expected total_time > 0, was: %d' % total_time)
     cpu_usage[process_type] = float(cpu_process_time) / total_time
   return cpu_usage