drive: Introduce DriveSystemServiceObserver

chrome/browser/chromeos/extensions/file_manager/file_manager_event_router.cc

Index: chrome/browser/chromeos/extensions/file_manager/file_manager_event_router.cc
diff --git a/chrome/browser/chromeos/extensions/file_manager/file_manager_event_router.cc b/chrome/browser/chromeos/extensions/file_manager/file_manager_event_router.cc
index 8a1c14cb8f952515f9518cd3f466d6194a9800bc..4cf9bd181165133e9ac9049aa34c2a1629ce3d94 100644
--- a/chrome/browser/chromeos/extensions/file_manager/file_manager_event_router.cc
+++ b/chrome/browser/chromeos/extensions/file_manager/file_manager_event_router.cc
@@ -293,6 +293,7 @@ void FileManagerEventRouter::Shutdown() {
   DriveSystemService* system_service =
       DriveSystemServiceFactory::FindForProfileRegardlessOfStates(profile_);
   if (system_service) {
+    system_service->RemoveObserver(this);
     system_service->file_system()->RemoveObserver(this);
     system_service->drive_service()->RemoveObserver(this);
   }
@@ -322,6 +323,7 @@ void FileManagerEventRouter::ObserveFileSystemEvents() {
   DriveSystemService* system_service =
       DriveSystemServiceFactory::GetForProfileRegardlessOfStates(profile_);
   if (system_service) {
+    system_service->AddObserver(this);
     system_service->drive_service()->AddObserver(this);
     system_service->file_system()->AddObserver(this);
   }