sync: Add interfaces for per-type sync

sync/engine/commit_contributor.cc

Index: sync/engine/commit_contributor.cc
diff --git a/sync/api/attachments/attachment_store.cc b/sync/engine/commit_contributor.cc
similarity index 61%
copy from sync/api/attachments/attachment_store.cc
copy to sync/engine/commit_contributor.cc
index e22b040e3dec3e691b35179b5dc3bae05bb18927..6410c63ccf89cfaecb0163864eca7a4398c5df2b 100644
--- a/sync/api/attachments/attachment_store.cc
+++ b/sync/engine/commit_contributor.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_contributor.h"
 
 namespace syncer {
 
-AttachmentStore::AttachmentStore() {}
-AttachmentStore::~AttachmentStore() {}
+CommitContributor::CommitContributor() {}
+
+CommitContributor::~CommitContributor() {}
 
 }  // namespace syncer