[Sync] Introduce AttachmentStoreForSync class

sync/internal_api/public/attachments/attachment_store_frontend.h

Index: sync/internal_api/public/attachments/attachment_store_frontend.h
diff --git a/sync/internal_api/public/attachments/attachment_store_frontend.h b/sync/internal_api/public/attachments/attachment_store_frontend.h
index ec4a33f24f661e381029968b56f333212e32238a..682cb4cb028823eef2565133476eedc0a4715e6c 100644
--- a/sync/internal_api/public/attachments/attachment_store_frontend.h
+++ b/sync/internal_api/public/attachments/attachment_store_frontend.h
@@ -40,18 +40,19 @@ class SYNC_EXPORT AttachmentStoreFrontend
   void Read(const AttachmentIdList& ids,
             const AttachmentStore::ReadCallback& callback);
 
-  void Write(AttachmentStore::AttachmentReferrer referrer,
+  void Write(AttachmentStore::Component component,
              const AttachmentList& attachments,
              const AttachmentStore::WriteCallback& callback);
-
-  void Drop(AttachmentStore::AttachmentReferrer referrer,
-            const AttachmentIdList& ids,
-            const AttachmentStore::DropCallback& callback);
+  void SetReference(AttachmentStore::Component component,
+                    const AttachmentIdList& ids);
+  void DropReference(AttachmentStore::Component component,
+                     const AttachmentIdList& ids,
+                     const AttachmentStore::DropCallback& callback);
 
   void ReadMetadata(const AttachmentIdList& ids,
                     const AttachmentStore::ReadMetadataCallback& callback);
 
-  void ReadAllMetadata(AttachmentStore::AttachmentReferrer referrer,
+  void ReadAllMetadata(AttachmentStore::Component component,
                        const AttachmentStore::ReadMetadataCallback& callback);
 
  private: