sync: Add interfaces for per-type sync

sync/engine/commit_contribution.cc

Index: sync/engine/commit_contribution.cc
diff --git a/sync/api/attachments/attachment_store.cc b/sync/engine/commit_contribution.cc
similarity index 60%
copy from sync/api/attachments/attachment_store.cc
copy to sync/engine/commit_contribution.cc
index e22b040e3dec3e691b35179b5dc3bae05bb18927..cef02b57b9021598370cc9e7554ab15042a9489a 100644
--- a/sync/api/attachments/attachment_store.cc
+++ b/sync/engine/commit_contribution.cc
@@ -2,11 +2,12 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#include "sync/api/attachments/attachment_store.h"
+#include "sync/engine/commit_contribution.h"
 
 namespace syncer {
 
-AttachmentStore::AttachmentStore() {}
-AttachmentStore::~AttachmentStore() {}
+CommitContribution::CommitContribution() {}
+
+CommitContribution::~CommitContribution() {}
 
 }  // namespace syncer