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

base/BUILD.gn

 # Copyright (c) 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("//build/config/ui.gni")
 
 if (is_android) {
   import("//build/config/android/rules.gni")
 }
 
@@ skipping 430 matching lines @@
     "power_monitor/power_monitor_device_source.cc",
     "power_monitor/power_monitor_device_source.h",
     "power_monitor/power_monitor_device_source_android.cc",
     "power_monitor/power_monitor_device_source_android.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 995 matching lines @@
 
   unittest_apk("base_unittests_apk") {
     deps = [
       ":base_java",
       ":base_java_unittest_support",
       ":base_unittests",
     ]
     unittests_dep = ":base_unittests"
   }
 }