[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..cc48d77374cec4397be925013a147bca89d4b388 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(
@@ -467,8 +463,7 @@ drive::DriveUploaderInterface* SyncEngine::GetDriveUploader() {
   return drive_uploader_.get();
 }
 
-MetadataDatabase* SyncEngine::GetMetadataDatabase() {
-  // TODO(peria): Post task
+MetadataDatabase* SyncEngine::GetMetadataDatabaseForTesting() {
   return sync_worker_->GetMetadataDatabase();
 }
 
@@ -516,11 +511,11 @@ void SyncEngine::UpdateServiceState(RemoteServiceState state,
       OnRemoteServiceStateUpdated(state, description));
 }
 
-void SyncEngine::UpdateRegisteredApps() {
+void SyncEngine::UpdateRegisteredAppsForTesting() {

[tzik, 2014/05/30 05:23:47]

Could you consolidate this to SyncWorker's one?

[peria, 2014/06/02 04:30:28]

Done.

   if (!extension_service_)
     return;
 
-  MetadataDatabase* metadata_db = GetMetadataDatabase();
+  MetadataDatabase* metadata_db = GetMetadataDatabaseForTesting();
   DCHECK(metadata_db);
   std::vector<std::string> app_ids;
   metadata_db->GetRegisteredAppIDs(&app_ids);