[SyncFS] Post tasks between SyncEngine and SyncWorker

chrome/browser/sync_file_system/drive_backend/sync_engine_context.cc

Index: chrome/browser/sync_file_system/drive_backend/sync_engine_context.cc
diff --git a/chrome/browser/sync_file_system/drive_backend/sync_engine_context.cc b/chrome/browser/sync_file_system/drive_backend/sync_engine_context.cc
index f87ab55fa7c1094ab56b6e3959bcd7b236219fd5..f9201e2646d5b1c419197b2f23f6c10176f8db20 100644
--- a/chrome/browser/sync_file_system/drive_backend/sync_engine_context.cc
+++ b/chrome/browser/sync_file_system/drive_backend/sync_engine_context.cc
@@ -19,11 +19,13 @@ namespace drive_backend {
 SyncEngineContext::SyncEngineContext(
     drive::DriveServiceInterface* drive_service,
     drive::DriveUploaderInterface* drive_uploader,
-    base::SequencedTaskRunner* task_runner)
+    base::SequencedTaskRunner* worker_task_runner,
+    base::SequencedTaskRunner* ui_task_runner)
     : drive_service_(drive_service),
       drive_uploader_(drive_uploader),
       remote_change_processor_(NULL),
-      task_runner_(task_runner) {}
+      worker_task_runner_(worker_task_runner),
+      ui_task_runner_(ui_task_runner) {}
 
 SyncEngineContext::~SyncEngineContext() {}
 
@@ -48,7 +50,11 @@ RemoteChangeProcessor* SyncEngineContext::GetRemoteChangeProcessor() {
 }
 
 base::SequencedTaskRunner* SyncEngineContext::GetBlockingTaskRunner() {
-  return task_runner_.get();
+  return worker_task_runner_.get();
+}
+
+base::SequencedTaskRunner* SyncEngineContext::GetUiTaskRunner() {
+  return ui_task_runner_.get();
 }
 
 void SyncEngineContext::SetMetadataDatabase(