Revert of Check to ensure PowerObservers are added and removed on the same thread

base/base.gypi

 # Copyright (c) 2012 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.
 
 {
   'target_defaults': {
     'variables': {
       'base_target': 0,
       'base_i18n_target': 0,
     },
@@ skipping 421 matching lines @@
           'power_monitor/power_monitor_device_source_android.cc',
           'power_monitor/power_monitor_device_source_android.h',
           'power_monitor/power_monitor_device_source.cc',
           'power_monitor/power_monitor_device_source.h',
           'power_monitor/power_monitor_device_source_ios.mm',
           'power_monitor/power_monitor_device_source_mac.mm',
           'power_monitor/power_monitor_device_source_posix.cc',
           'power_monitor/power_monitor_device_source_win.cc',
           'power_monitor/power_monitor_source.cc',
           'power_monitor/power_monitor_source.h',
-          'power_monitor/power_observer.cc',
           'power_monitor/power_observer.h',
           'process/internal_linux.cc',
           'process/internal_linux.h',
           'process/kill.cc',
           'process/kill.h',
           'process/kill_mac.cc',
           'process/kill_posix.cc',
           'process/kill_win.cc',
           'process/launch.cc',
           'process/launch.h',
@@ skipping 545 matching lines @@
           'i18n/time_formatting.h',
           'i18n/timezone.cc',
           'i18n/timezone.h',
           'i18n/utf8_validator_tables.cc',
           'i18n/utf8_validator_tables.h',
         ],
       }]
     ],
   },
 }