Add AttachmentDownloader interface, change signature of AttachmentStore::Read

sync/api/attachments/attachment_downloader.cc

Index: sync/api/attachments/attachment_downloader.cc
diff --git a/sync/engine/update_handler.cc b/sync/api/attachments/attachment_downloader.cc
similarity index 65%
copy from sync/engine/update_handler.cc
copy to sync/api/attachments/attachment_downloader.cc
index 308fee95bb1074c197b6973f38b49eb017d9ba7c..a0108f70e8941f26b4cbd4bb9429dd5432216207 100644
--- a/sync/engine/update_handler.cc
+++ b/sync/api/attachments/attachment_downloader.cc
@@ -2,13 +2,11 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#include "sync/engine/update_handler.h"
+#include "sync/api/attachments/attachment_downloader.h"
 
 namespace syncer {
 
-UpdateHandler::UpdateHandler() {}
-
-UpdateHandler::~UpdateHandler() {}
-
+AttachmentDownloader::~AttachmentDownloader() {
+}
 
 }  // namespace syncer