Improve readability of power monitoring code and add additional

tools/telemetry/telemetry/internal/platform/power_monitor/powermetrics_power_monitor.py

Index: tools/telemetry/telemetry/internal/platform/power_monitor/powermetrics_power_monitor.py
diff --git a/tools/telemetry/telemetry/internal/platform/power_monitor/powermetrics_power_monitor.py b/tools/telemetry/telemetry/internal/platform/power_monitor/powermetrics_power_monitor.py
index f43578c74712c046b6014e43b43db713101e500d..d800e6095a1511a430e50e21f9002f7398bcb9a1 100644
--- a/tools/telemetry/telemetry/internal/platform/power_monitor/powermetrics_power_monitor.py
+++ b/tools/telemetry/telemetry/internal/platform/power_monitor/powermetrics_power_monitor.py
@@ -16,6 +16,7 @@ from telemetry import decorators
 from telemetry.internal.platform import power_monitor
 
 
+# TODO: rename this class (seems like this is used by mac)
 class PowerMetricsPowerMonitor(power_monitor.PowerMonitor):
 
   def __init__(self, backend):
@@ -30,8 +31,7 @@ class PowerMetricsPowerMonitor(power_monitor.PowerMonitor):
     return '/usr/bin/powermetrics'
 
   def StartMonitoringPower(self, browser):
-    assert not self._powermetrics_process, (
-        'Must call StopMonitoringPower().')
+    self._CheckStart()
     # Empirically powermetrics creates an empty output file immediately upon
     # starting.  We detect file creation as a signal that measurement has
     # started.  In order to avoid various race conditions in tempfile creation
@@ -254,8 +254,7 @@ class PowerMetricsPowerMonitor(power_monitor.PowerMonitor):
           elevate_privilege=True)
 
   def StopMonitoringPower(self):
-    assert self._powermetrics_process, (
-        'StartMonitoringPower() not called.')
+    self._CheckStop()
     # Tell powermetrics to take an immediate sample.
     try:
       self._KillPowerMetricsProcess()