Reland of Migrate DeviceUtils.ReadFile to adb_wrapper (try 3)

tools/telemetry/telemetry/core/backends/android_command_line_backend.py

 # Copyright 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import logging
 import pipes
 import sys
 
 from telemetry.core import util
 
@@ skipping 45 matching lines @@
   particular features or modify otherwise default functionality. To set the
   flags for Chrome on Android, specific files on the device must be updated
   with the flags to enable. This class provides a wrapper around this
   functionality.
   """
 
   def __init__(self, adb, backend_settings, startup_args):
     self._adb = adb
     self._backend_settings = backend_settings
     self._startup_args = startup_args
-    self._saved_command_line_file_contents = ''
+    self._saved_command_line_file_contents = None
 
   @property
   def command_line_file(self):
     return self._backend_settings.GetCommandLineFile(self._adb.IsUserBuild())
 
   def SetUpCommandLineFlags(self):
     args = [self._backend_settings.pseudo_exec_name]
     args.extend(self._startup_args)
     content = ' '.join(_QuoteIfNeeded(arg) for arg in args)
 
     try:
       # Save the current command line to restore later, except if it appears to
       # be a  Telemetry created one. This is to prevent a common bug where
       # --host-resolver-rules borks people's browsers if something goes wrong
       # with Telemetry.
-      self._saved_command_line_file_contents = ''.join(self._ReadFile())
+      self._saved_command_line_file_contents = self._ReadFile()
       if '--host-resolver-rules' in self._saved_command_line_file_contents:
-        self._saved_command_line_file_contents = ''
+        self._saved_command_line_file_contents = None
+    except device_errors.CommandFailedError:
+      self._saved_command_line_file_contents = None
+
+    try:
       self._WriteFile(content)
-    except device_errors.CommandFailedError:
+    except device_errors.CommandFailedError as exc:
+      logging.critical(exc)
       logging.critical('Cannot set Chrome command line. '
                        'Fix this by flashing to a userdebug build.')
       sys.exit(1)
 
   def RestoreCommandLineFlags(self):
-    self._WriteFile(self._saved_command_line_file_contents)
+    if self._saved_command_line_file_contents is None:
+      self._RemoveFile()
+    else:
+      self._WriteFile(self._saved_command_line_file_contents)
 
   def _ReadFile(self):
-    return self._adb.device().ReadFile(self.command_line_file)
+    return self._adb.device().ReadFile(self.command_line_file, as_root=True)
 
   def _WriteFile(self, contents):
     self._adb.device().WriteFile(self.command_line_file, contents, as_root=True)
+
+  def _RemoveFile(self):
+    self._adb.device().RunShellCommand(['rm', '-f', self.command_line_file],
+                                       as_root=True, check_return=True)