Revert of [SyncFS] Post tasks between SyncEngine and SyncWorker

chrome/browser/sync_file_system/drive_backend/local_to_remote_syncer.cc

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/sync_file_system/drive_backend/local_to_remote_syncer.h"
 
 #include <string>
 #include <vector>
 
 #include "base/callback.h"
@@ skipping 366 matching lines @@
   }
   callback.Run(SYNC_STATUS_OK);
 }
 
 void LocalToRemoteSyncer::UploadExistingFile(
     const SyncStatusCallback& callback)  {
   DCHECK(remote_file_tracker_);
   DCHECK(remote_file_tracker_->has_synced_details());
 
   base::PostTaskAndReplyWithResult(
-      sync_context_->GetFileTaskRunner(), FROM_HERE,
+      sync_context_->GetBlockingTaskRunner(), FROM_HERE,
       base::Bind(&drive::util::GetMd5Digest, local_path_),
       base::Bind(&LocalToRemoteSyncer::DidGetMD5ForUpload,
                  weak_ptr_factory_.GetWeakPtr(),
                  callback));
 }
 
 void LocalToRemoteSyncer::DidGetMD5ForUpload(
     const SyncStatusCallback& callback,
     const std::string& local_file_md5) {
   if (local_file_md5 == remote_file_tracker_->synced_details().md5()) {
@@ skipping 287 matching lines @@
   set_used_network(true);
   return sync_context_->GetDriveUploader();
 }
 
 MetadataDatabase* LocalToRemoteSyncer::metadata_database() {
   return sync_context_->GetMetadataDatabase();
 }
 
 }  // namespace drive_backend
 }  // namespace sync_file_system