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

sync/internal_api/public/attachments/attachment_service_proxy.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_ATTACHMENT_SERVICE_PROXY_H_
 #define SYNC_INTERNAL_API_PUBLIC_ATTACHMENTS_ATTACHMENT_SERVICE_PROXY_H_
 
 #include "base/basictypes.h"
 #include "base/callback.h"
 #include "base/memory/scoped_ptr.h"
@@ skipping 32 matching lines @@
   // |wrapped_task_runner| thread.
   //
   // Note, this object does not own |wrapped|.  When |wrapped| is destroyed,
   // calls to this object become no-ops.
   AttachmentServiceProxy(
       const scoped_refptr<base::SequencedTaskRunner>& wrapped_task_runner,
       const base::WeakPtr<syncer::AttachmentService>& wrapped);
 
   ~AttachmentServiceProxy() override;
 
-  // AttachmentService implementation.
-  //
-  // GetStore always returns NULL.
-  AttachmentStore* GetStore() override;
   void GetOrDownloadAttachments(const AttachmentIdList& attachment_ids,
                                 const GetOrDownloadCallback& callback) override;
   void UploadAttachments(const AttachmentIdSet& attachment_ids) override;
 
  protected:
   // Core does the work of proxying calls to AttachmentService methods from one
   // thread to another so AttachmentServiceProxy can be an easy-to-use,
   // non-ref-counted A ref-counted class.
   //
   // Callback from AttachmentService are proxied back using free functions
   // defined in the .cc file (ProxyFooCallback functions).
   //
   // Core is ref-counted because we want to allow AttachmentServiceProxy to be
   // copy-constructable while allowing for different implementations of Core
   // (e.g. one type of core might own the wrapped AttachmentService).
   //
   // Calls to objects of this class become no-ops once its wrapped object is
   // destroyed.
   class SYNC_EXPORT Core : public AttachmentService,
                            public base::RefCountedThreadSafe<Core> {
    public:
     // Construct an AttachmentServiceProxyCore that forwards calls to |wrapped|.
     Core(const base::WeakPtr<syncer::AttachmentService>& wrapped);
 
     // AttachmentService implementation.
-    AttachmentStore* GetStore() override;
     void GetOrDownloadAttachments(
         const AttachmentIdList& attachment_ids,
         const GetOrDownloadCallback& callback) override;
     void UploadAttachments(const AttachmentIdSet& attachment_ids) override;
 
    protected:
     friend class base::RefCountedThreadSafe<Core>;
     ~Core() override;
 
    private:
     base::WeakPtr<AttachmentService> wrapped_;
 
     DISALLOW_COPY_AND_ASSIGN(Core);
   };
 
   // Used in tests to create an AttachmentServiceProxy with a custom Core.
   AttachmentServiceProxy(
       const scoped_refptr<base::SequencedTaskRunner>& wrapped_task_runner,
       const scoped_refptr<Core>& core);
 
  private:
   scoped_refptr<base::SequencedTaskRunner> wrapped_task_runner_;
   scoped_refptr<Core> core_;
 };
 
 }  // namespace syncer
 
 #endif  // SYNC_INTERNAL_API_PUBLIC_ATTACHMENTS_ATTACHMENT_SERVICE_PROXY_H_