Add a ClientAction used by SyncRollbackManager to notify PSS that rollback is

sync/internal_api/sync_rollback_manager_base.cc

Index: sync/internal_api/sync_rollback_manager_base.cc
diff --git a/sync/internal_api/sync_rollback_manager_base.cc b/sync/internal_api/sync_rollback_manager_base.cc
index e5a4779685f1aec773635c7f05043341d3db05b2..af203410ae24ffd49ea84e7cac0c8266a690e4ad 100644
--- a/sync/internal_api/sync_rollback_manager_base.cc
+++ b/sync/internal_api/sync_rollback_manager_base.cc
@@ -332,6 +332,10 @@ void SyncRollbackManagerBase::InitBookmarkFolder(const std::string& folder) {
   entry.PutSpecifics(specifics);
 }
 
+ObserverList<SyncManager::Observer>* SyncRollbackManagerBase::GetObservers() {
+  return &observers_;
+}
+
 void SyncRollbackManagerBase::RegisterDirectoryTypeDebugInfoObserver(
     syncer::TypeDebugInfoObserver* observer) {}