Use ScopedObserver to observe the SupervisedUserSyncService in SupervisedUserImportHandler.

In addition to simplifying the code, ScopedObserver ensures that the object is unregistered as an observer if and only if it has been registered.

BUG=488642

Review URL: https://codereview.chromium.org/1123283011

Cr-Commit-Position: refs/heads/master@{#330536}

chrome/browser/ui/webui/options/supervised_user_import_handler.cc

@@ -56,19 +56,12 @@ namespace options {
 SupervisedUserImportHandler::SupervisedUserImportHandler()
     : profile_observer_(this),
       signin_error_observer_(this),
+      supervised_user_sync_service_observer_(this),
       removed_profile_is_supervised_(false),
       weak_ptr_factory_(this) {
 }
 
 SupervisedUserImportHandler::~SupervisedUserImportHandler() {
-  Profile* profile = Profile::FromWebUI(web_ui());
-  if (!profile->IsSupervised()) {
-    SupervisedUserSyncService* service =
-        SupervisedUserSyncServiceFactory::GetForProfile(profile);
-    if (service)
-      service->RemoveObserver(this);
-    subscription_.reset();
-  }
 }
 
 void SupervisedUserImportHandler::GetLocalizedValues(
@@ -105,7 +98,7 @@ void SupervisedUserImportHandler::InitializeHandler() {
     SupervisedUserSyncService* sync_service =
         SupervisedUserSyncServiceFactory::GetForProfile(profile);
     if (sync_service) {
-      sync_service->AddObserver(this);
+      supervised_user_sync_service_observer_.Add(sync_service);
       signin_error_observer_.Add(
           SigninErrorControllerFactory::GetForProfile(profile));
       SupervisedUserSharedSettingsService* settings_service =

chrome/browser/ui/webui/options/supervised_user_import_handler.h

@@ -22,6 +22,7 @@ class ListValue;
 }
 
 class ProfileInfoCache;
+class SupervisedUserSyncService;
 
 namespace options {
 
@@ -100,6 +101,8 @@ class SupervisedUserImportHandler : public OptionsPageUIHandler,
       profile_observer_;
   ScopedObserver<SigninErrorController, SupervisedUserImportHandler>
       signin_error_observer_;
+  ScopedObserver<SupervisedUserSyncService, SupervisedUserImportHandler>
+      supervised_user_sync_service_observer_;
 
   bool removed_profile_is_supervised_;