[Android] Enable root in perf_control.SetHighPerfMode.

SetHighPerfMode needs root, but it currently only checks to see if
we already have root rather than trying to enable it.

BUG=478104

Review URL: https://codereview.chromium.org/1135393005

Cr-Commit-Position: refs/heads/master@{#329980}

build/android/pylib/perf/perf_control.py

@@ -6,8 +6,10 @@ import atexit
 import logging
 
 from pylib import android_commands
+from pylib.device import device_errors
 from pylib.device import device_utils
 
+
 class PerfControl(object):
   """Provides methods for setting the performance mode of a device."""
   _CPU_PATH = '/sys/devices/system/cpu'
@@ -28,7 +30,9 @@ class PerfControl(object):
 
   def SetHighPerfMode(self):
     """Sets the highest stable performance mode for the device."""
-    if not self._device.HasRoot():
+    try:
+      self._device.EnableRoot()
+    except device_errors.CommandFailedError:
       message = 'Need root for performance mode. Results may be NOISY!!'
       logging.warning(message)
       # Add an additional warning at exit, such that it's clear that any results