Telemetry: fix cpu metric.

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.
 
 from metrics import Metric
 
 class CpuMetric(Metric):
   """Calulates CPU load over a span of time."""
 
   def __init__(self, browser):
     super(CpuMetric, self).__init__()
     self._results = None
     self._browser = browser
     self._start_cpu = None
 
   def DidStartBrowser(self, browser):
     # Save the browser object so that cpu_stats can be accessed later.
     self._browser = browser
 
   def Start(self, page, tab):
     self._start_cpu = self._browser.cpu_stats
 
   def Stop(self, page, tab):
     assert self._start_cpu, 'Must call Start() first'
-    self._results = CpuMetric.SubtractCpuStats(self._browser.cpu_stats,
-                                         self.start_cpu)
+    self._results = _SubtractCpuStats(self._browser.cpu_stats, self._start_cpu)
 
   # Optional argument trace_name is not in base class Metric.
   # pylint: disable=W0221
   def AddResults(self, tab, results, trace_name='cpu_utilization'):
     assert self._results, 'Must call Stop() first'
     # Add a result for each process type.
     for process_type in self._results:
       trace_name = '%s_%s' % (trace_name, process_type.lower())
       cpu_percent = 100 * self._results[process_type]
       results.Add(trace_name, '%', cpu_percent, chart_name='cpu_utilization',
@@ skipping 23 matching lines @@
     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'])
     assert total_time > 0
     cpu_usage[process_type] = float(cpu_process_time) / total_time
   return cpu_usage