Fix potential crashes in NetworkHandler code

chrome/browser/chromeos/login/screens/network_screen.cc

 // 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.
 
 #include "chrome/browser/chromeos/login/screens/network_screen.h"
 
 #include <utility>
 
 #include "base/location.h"
 #include "base/logging.h"
@@ skipping 100 matching lines @@
 void NetworkScreen::Initialize(::login::ScreenContext* context) {
   NetworkModel::Initialize(context);
   OnSystemTimezoneChanged();
   UpdateLanguageList();
 }
 
 void NetworkScreen::OnViewDestroyed(NetworkView* view) {
   if (view_ == view) {
     view_ = nullptr;
     timezone_subscription_.reset();
+    // Ownership of NetworkScreen is complicated; ensure that we remove
+    // this as a NetworkStateHandler observer when the view is destroyed.
+    UnsubscribeNetworkNotification();
   }
 }
 
 void NetworkScreen::OnUserAction(const std::string& action_id) {
   if (action_id == kUserActionContinueButtonClicked) {
     OnContinueButtonPressed();
   } else if (action_id == kUserActionConnectDebuggingFeaturesClicked) {
     if (delegate_)
       delegate_->OnEnableDebuggingScreenRequested();
   } else {
@@ skipping 306 matching lines @@
   FOR_EACH_OBSERVER(Observer, observers_, OnLanguageListReloaded());
 }
 
 void NetworkScreen::OnSystemTimezoneChanged() {
   std::string current_timezone_id;
   CrosSettings::Get()->GetString(kSystemTimezone, &current_timezone_id);
   GetContextEditor().SetString(kContextKeyTimezone, current_timezone_id);
 }
 
 }  // namespace chromeos