telemetry: Fix runaway subprocess in monsoon_power_monitor.py

Telemetry leaves a memory hogging subprocess behind if:
1) A Monsoon Power Monitor is attached to host computer (Linux),
2) User runs a power-metric-using benchmark such as sunspider on Android
   ChromeShell, and
3) Benchmark raises an exception between power metric's Start and Stop
   calls. For example, an exception will occur if browser crashes and
   the benchmark does something like tab.WaitForJavaScriptExpression().

This patch fixes the issue by setting the daemon flag on the subprocess.
This will cause the main process to kill the subprocess on exit.

BUG=413596

R=ernstm@chromium.org

Review URL: https://codereview.chromium.org/572643002

Cr-Commit-Position: refs/heads/master@{#296177}

AUTHORS

@@ -450,6 +450,7 @@ Victor Costan <costan@gmail.com>
 Viet-Trung Luu <viettrungluu@gmail.com>
 Vinay Anantharaman <vinaya@adobe.com>
 Vipul Bhasin <vipul.bhasin@gmail.com>
+Visa Putkinen <v.putkinen@partner.samsung.com>
 Vivek Galatage <vivek.vg@samsung.com>
 Will Hirsch <chromium@willhirsch.co.uk>
 Xiang Long <xiang.long@intel.com>

tools/telemetry/telemetry/core/platform/power_monitor/monsoon_power_monitor.py

@@ -73,6 +73,8 @@ class MonsoonPowerMonitor(sysfs_power_monitor.SysfsPowerMonitor):
         args=(self._monsoon,
               self._is_collecting,
               self._powermonitor_output_file))
+    # Ensure child is not left behind: parent kills daemonic children on exit.
+    self._powermonitor_process.daemon = True
     self._powermonitor_process.start()
     if not self._is_collecting.wait(timeout=0.5):
       self._powermonitor_process.terminate()