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

sync/internal_api/public/attachments/in_memory_attachment_store.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef SYNC_INTERNAL_API_PUBLIC_ATTACHMENTS_IN_MEMORY_ATTACHMENT_STORE_H_
 #define SYNC_INTERNAL_API_PUBLIC_ATTACHMENTS_IN_MEMORY_ATTACHMENT_STORE_H_
 
 #include "base/memory/ref_counted.h"
 #include "base/threading/non_thread_safe.h"
 #include "sync/api/attachments/attachment.h"
 #include "sync/api/attachments/attachment_id.h"
 #include "sync/api/attachments/attachment_store.h"
+#include "sync/api/attachments/attachment_store_backend.h"
 #include "sync/base/sync_export.h"
 
 namespace base {
 class SequencedTaskRunner;
 }  // namespace base
 
 namespace syncer {
 
 // An in-memory implementation of AttachmentStore used for testing.
 // InMemoryAttachmentStore is not threadsafe, it lives on backend thread and
 // posts callbacks with results on |callback_task_runner|.
 class SYNC_EXPORT InMemoryAttachmentStore : public AttachmentStoreBackend,
                                             public base::NonThreadSafe {
  public:
   InMemoryAttachmentStore(
       const scoped_refptr<base::SequencedTaskRunner>& callback_task_runner);
   ~InMemoryAttachmentStore() override;
 
   // AttachmentStoreBackend implementation.
   void Init(const AttachmentStore::InitCallback& callback) override;
   void Read(const AttachmentIdList& ids,
             const AttachmentStore::ReadCallback& callback) override;
-  void Write(const AttachmentList& attachments,
+  void Write(AttachmentStore::AttachmentReferrer referrer,
+             const AttachmentList& attachments,
              const AttachmentStore::WriteCallback& callback) override;
-  void Drop(const AttachmentIdList& ids,
+  void Drop(AttachmentStore::AttachmentReferrer referrer,
+            const AttachmentIdList& ids,
             const AttachmentStore::DropCallback& callback) override;
   void ReadMetadata(
       const AttachmentIdList& ids,
       const AttachmentStore::ReadMetadataCallback& callback) override;
   void ReadAllMetadata(
+      AttachmentStore::AttachmentReferrer referrer,
       const AttachmentStore::ReadMetadataCallback& callback) override;
 
  private:
   AttachmentMap attachments_;
 
   DISALLOW_COPY_AND_ASSIGN(InMemoryAttachmentStore);
 };
 
 }  // namespace syncer
 
 #endif  // SYNC_INTERNAL_API_PUBLIC_ATTACHMENTS_IN_MEMORY_ATTACHMENT_STORE_H_