Improve readability of power monitoring code and add additional

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

Index: tools/telemetry/telemetry/internal/platform/power_monitor/__init__.py
diff --git a/tools/telemetry/telemetry/internal/platform/power_monitor/__init__.py b/tools/telemetry/telemetry/internal/platform/power_monitor/__init__.py
index 7a13fa91d2b2de33e6024e89547bafc3ba7c6e15..4c3cf4532cbe3859ba6d966d185a45b76ef7d041 100644
--- a/tools/telemetry/telemetry/internal/platform/power_monitor/__init__.py
+++ b/tools/telemetry/telemetry/internal/platform/power_monitor/__init__.py
@@ -2,6 +2,8 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import logging
+
 from telemetry.core import exceptions
 
 
@@ -10,6 +12,9 @@ class PowerMonitor(object):
 
   Provides an interface to register power consumption during a test.
   """
+  def __init__(self):
+    self._monitoring = False
+
   def CanMonitorPower(self):
     """Returns True iff power can be monitored asynchronously via
     StartMonitoringPower() and StopMonitoringPower().
@@ -22,6 +27,14 @@ class PowerMonitor(object):
     energy consumption."""
     return False
 
+  def _CheckStart(self):
+    assert not self._monitoring, "Already monitoring power."
+    self._monitoring = True
+
+  def _CheckStop(self):
+    assert self._monitoring, "Not monitoring power."
+    self._monitoring = False
+
   def StartMonitoringPower(self, browser):
     """Starts monitoring power utilization statistics.