Revert of [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 b8cadb9fc0441db7a4cc0ed6fdf1f93406c753ab..f87ab55fa7c1094ab56b6e3959bcd7b236219fd5 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
@@ -19,13 +19,11 @@
 SyncEngineContext::SyncEngineContext(
     drive::DriveServiceInterface* drive_service,
     drive::DriveUploaderInterface* drive_uploader,
-    base::SequencedTaskRunner* ui_task_runner,
-    base::SequencedTaskRunner* file_task_runner)
+    base::SequencedTaskRunner* task_runner)
     : drive_service_(drive_service),
       drive_uploader_(drive_uploader),
       remote_change_processor_(NULL),
-      ui_task_runner_(ui_task_runner),
-      file_task_runner_(file_task_runner) {}
+      task_runner_(task_runner) {}
 
 SyncEngineContext::~SyncEngineContext() {}
 
@@ -49,12 +47,8 @@
   return remote_change_processor_;
 }
 
-base::SequencedTaskRunner* SyncEngineContext::GetUiTaskRunner() {
-  return ui_task_runner_.get();
-}
-
-base::SequencedTaskRunner* SyncEngineContext::GetFileTaskRunner() {
-  return file_task_runner_.get();
+base::SequencedTaskRunner* SyncEngineContext::GetBlockingTaskRunner() {
+  return task_runner_.get();
 }
 
 void SyncEngineContext::SetMetadataDatabase(