[SyncFS] Make routines using metadata database async

chrome/browser/sync_file_system/drive_backend/sync_engine.cc

Index: chrome/browser/sync_file_system/drive_backend/sync_engine.cc
diff --git a/chrome/browser/sync_file_system/drive_backend/sync_engine.cc b/chrome/browser/sync_file_system/drive_backend/sync_engine.cc
index 9dc0c8181bcdcc349ff1e47a07d705178251241a..1590d18b82e8f7a3fd01798ec3e5dbdd1d214722 100644
--- a/chrome/browser/sync_file_system/drive_backend/sync_engine.cc
+++ b/chrome/browser/sync_file_system/drive_backend/sync_engine.cc
@@ -391,14 +391,10 @@ void SyncEngine::SetSyncEnabled(bool enabled) {
 }
 
 void SyncEngine::PromoteDemotedChanges() {
-  MetadataDatabase* metadata_db = GetMetadataDatabase();
-  if (metadata_db && metadata_db->HasLowPriorityDirtyTracker()) {
-    metadata_db->PromoteLowerPriorityTrackersToNormal();
-    FOR_EACH_OBSERVER(
-        Observer,
-        service_observers_,
-        OnRemoteChangeQueueUpdated(metadata_db->CountDirtyTracker()));
-  }
+  worker_task_runner_->PostTask(
+      FROM_HERE,
+      base::Bind(&SyncWorker::PromoteDemotedChanges,
+                 base::Unretained(sync_worker_.get())));
 }
 
 void SyncEngine::ApplyLocalChange(
@@ -419,10 +415,6 @@ void SyncEngine::ApplyLocalChange(
                      FROM_HERE, callback)));
 }
 
-SyncTaskManager* SyncEngine::GetSyncTaskManagerForTesting() {
-  return sync_worker_->GetSyncTaskManager();
-}
-
 void SyncEngine::OnNotificationReceived() {
   worker_task_runner_->PostTask(
       FROM_HERE,
@@ -467,11 +459,6 @@ drive::DriveUploaderInterface* SyncEngine::GetDriveUploader() {
   return drive_uploader_.get();
 }
 
-MetadataDatabase* SyncEngine::GetMetadataDatabase() {
-  // TODO(peria): Post task
-  return sync_worker_->GetMetadataDatabase();
-}
-
 SyncEngine::SyncEngine(
     scoped_ptr<drive::DriveServiceInterface> drive_service,
     scoped_ptr<drive::DriveUploaderInterface> drive_uploader,
@@ -516,11 +503,11 @@ void SyncEngine::UpdateServiceState(RemoteServiceState state,
       OnRemoteServiceStateUpdated(state, description));
 }
 
-void SyncEngine::UpdateRegisteredApps() {
+void SyncEngine::UpdateRegisteredAppsForTesting() {
   if (!extension_service_)
     return;
 
-  MetadataDatabase* metadata_db = GetMetadataDatabase();
+  MetadataDatabase* metadata_db = sync_worker_->GetMetadataDatabase();
   DCHECK(metadata_db);
   std::vector<std::string> app_ids;
   metadata_db->GetRegisteredAppIDs(&app_ids);