Add AttachmentDownloader interface, change signature of AttachmentStore::Read

sync/api/attachments/attachment_uploader.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_API_ATTACHMENTS_ATTACHMENT_UPLOADER_H_
 #define SYNC_API_ATTACHMENTS_ATTACHMENT_UPLOADER_H_
 
 #include "base/basictypes.h"
 #include "base/callback.h"
 #include "base/memory/scoped_ptr.h"
-#include "base/memory/weak_ptr.h"
 #include "sync/api/attachments/attachment.h"
 #include "sync/base/sync_export.h"
 
 namespace syncer {
 
 // AttachmentUploader is responsible for uploading attachments to the server.
 class SYNC_EXPORT AttachmentUploader {
  public:
   // The result of an UploadAttachment operation.
   enum UploadResult {
     UPLOAD_SUCCESS,            // No error, attachment was uploaded
                                // successfully.
     UPLOAD_UNSPECIFIED_ERROR,  // An unspecified error occurred.
   };
 
   typedef base::Callback<void(const UploadResult&, const AttachmentId&)>
       UploadCallback;
 
   AttachmentUploader();
   virtual ~AttachmentUploader();
 
   // Upload |attachment| and invoke |callback| when done.
   //
   // |callback| will be invoked when the operation has completed (successfully
   // or otherwise).
   //
   // |callback| will receive an UploadResult code and an updated AttachmentId
-  // |containing the server address of the newly uploaded attachment.
+  // containing the server address of the newly uploaded attachment.
   virtual void UploadAttachment(const Attachment& attachment,
                                 const UploadCallback& callback) = 0;
 };
 
 }  // namespace syncer
 
 #endif  // SYNC_API_ATTACHMENTS_ATTACHMENT_UPLOADER_H_