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

This behavior is required by ObserverListThreadSafe and may leads to crashes if not followed

BUG=404767

Review URL: https://codereview.chromium.org/502003003

Cr-Commit-Position: refs/heads/master@{#293230}

base/BUILD.gn

@@ -448,6 +448,7 @@ component("base") {
     "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",

base/base.gypi

@@ -439,6 +439,7 @@
           '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',

base/power_monitor/power_monitor.cc

@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 #include "base/power_monitor/power_monitor.h"
+#include "base/logging.h"
 #include "base/power_monitor/power_monitor_source.h"
 
 namespace base {
@@ -27,11 +28,17 @@ PowerMonitor* PowerMonitor::Get() {
 }
 
 void PowerMonitor::AddObserver(PowerObserver* obs) {
+  DCHECK(!obs->power_monitor_thread_checker_);
+  obs->power_monitor_thread_checker_.reset(new base::ThreadChecker());
   observers_->AddObserver(obs);
 }
 
 void PowerMonitor::RemoveObserver(PowerObserver* obs) {
+  // PowerObservers must be removed on the same thread on which they were added.
+  DCHECK(obs->power_monitor_thread_checker_);
+  DCHECK(obs->power_monitor_thread_checker_->CalledOnValidThread());
   observers_->RemoveObserver(obs);
+  obs->power_monitor_thread_checker_.reset();
 }
 
 PowerMonitorSource* PowerMonitor::Source() {

base/power_monitor/power_observer.cc

+// Copyright 2014 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.
+
+#include "base/power_monitor/power_observer.h"
+
+namespace base {
+
+PowerObserver::PowerObserver() {
+}
+
+PowerObserver::~PowerObserver() {
+}
+
+}  // namespace base

base/power_monitor/power_observer.h

@@ -7,14 +7,20 @@
 
 #include "base/base_export.h"
 #include "base/compiler_specific.h"
+#include "base/memory/scoped_ptr.h"
+#include "base/threading/thread_checker.h"
 
 namespace base {
 
 class BASE_EXPORT PowerObserver {
+  friend class PowerMonitor;
+
  public:
+  PowerObserver();
+
   // Notification of a change in power status of the computer, such
   // as from switching between battery and A/C power.
-  virtual void OnPowerStateChange(bool on_battery_power) {};
+  virtual void OnPowerStateChange(bool on_battery_power) {}
 
   // Notification that the system is suspending.
   virtual void OnSuspend() {}
@@ -23,7 +29,12 @@ class BASE_EXPORT PowerObserver {
   virtual void OnResume() {}
 
  protected:
-  virtual ~PowerObserver() {}
+  virtual ~PowerObserver();
+
+ private:
+  // Allows PowerMonitor to ensure this observer is added and removed
+  // from the same thread, which is required by the ObserverListThreadSafe.
+  scoped_ptr<base::ThreadChecker> power_monitor_thread_checker_;
 };
 
 }  // namespace base