[Sync] Refactor AttachmentStore classes. Introduce concept of referrer.

sync/internal_api/attachments/on_disk_attachment_store.cc

Index: sync/internal_api/attachments/on_disk_attachment_store.cc
diff --git a/sync/internal_api/attachments/on_disk_attachment_store.cc b/sync/internal_api/attachments/on_disk_attachment_store.cc
index 7b715e03b7e7b254ecf6db283f285d218be9808f..ec6fc75931bd9283d50033f4b055f9f52f514717 100644
--- a/sync/internal_api/attachments/on_disk_attachment_store.cc
+++ b/sync/internal_api/attachments/on_disk_attachment_store.cc
@@ -134,6 +134,7 @@ void OnDiskAttachmentStore::Read(
 }
 
 void OnDiskAttachmentStore::Write(
+    AttachmentStore::AttachmentReferrer referrer,
     const AttachmentList& attachments,
     const AttachmentStore::WriteCallback& callback) {
   DCHECK(CalledOnValidThread());
@@ -153,6 +154,7 @@ void OnDiskAttachmentStore::Write(
 }
 
 void OnDiskAttachmentStore::Drop(
+    AttachmentStore::AttachmentReferrer referrer,
     const AttachmentIdList& ids,
     const AttachmentStore::DropCallback& callback) {
   DCHECK(CalledOnValidThread());
@@ -206,6 +208,7 @@ void OnDiskAttachmentStore::ReadMetadata(
 }
 
 void OnDiskAttachmentStore::ReadAllMetadata(
+    AttachmentStore::AttachmentReferrer referrer,
     const AttachmentStore::ReadMetadataCallback& callback) {
   DCHECK(CalledOnValidThread());
   AttachmentStore::Result result_code =