[Telemetry] Add power usage metric

tools/telemetry/telemetry/core/platform/mac_platform_backend.py

Index: tools/telemetry/telemetry/core/platform/mac_platform_backend.py
diff --git a/tools/telemetry/telemetry/core/platform/mac_platform_backend.py b/tools/telemetry/telemetry/core/platform/mac_platform_backend.py
index a427a7e692b968b7f3de48be18aa46c452bb6a94..fdace990343e4d6c4c3165f8fc61b659ce2e7631 100644
--- a/tools/telemetry/telemetry/core/platform/mac_platform_backend.py
+++ b/tools/telemetry/telemetry/core/platform/mac_platform_backend.py
@@ -3,6 +3,7 @@
 # found in the LICENSE file.
 
 import ctypes
+import logging
 import os
 import plistlib
 import signal
@@ -35,9 +36,15 @@ class MacPlatformBackend(posix_platform_backend.PosixPlatformBackend):
       self._powermetrics_output_file = tempfile.NamedTemporaryFile().name
       args = ['sudo', self.binary_path, '-f', 'plist', '-i',
           '%d' % SAMPLE_INTERVAL_MS, '-u', self._powermetrics_output_file]
-      # TODO(jeremy): Need to ensure command is run as root user.
+
+      # powermetrics writes lots of output to stderr, don't echo unless verbose
+      # logging enabled.
+      stderror_destination = subprocess.PIPE
+      if logging.getLogger().isEnabledFor(logging.INFO):

[tonyg, 2014/01/23 19:11:24]

The bots enable INFO, and we probably don't want the spam there, right? Should
this be DEBUG instead?

+        stderror_destination = None
+
       self._powermetrics_process = subprocess.Popen(args,
-          stdout=subprocess.PIPE)
+          stdout=subprocess.PIPE, stderr=stderror_destination)
 
     def _SendPrivilegedSignal(self, pid, signal_num):
       args = ['sudo', 'kill', '-%d' % signal_num, '%d' % pid]