Add AttachmentDownloader interface, change signature of AttachmentStore::Read

sync/api/attachments/attachment_downloader.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_DOWNLOADER_H_
+#define SYNC_API_ATTACHMENTS_ATTACHMENT_DOWNLOADER_H_
+
+#include "base/callback.h"
+#include "base/memory/scoped_ptr.h"
+#include "sync/api/attachments/attachment.h"
+#include "sync/base/sync_export.h"
+
+namespace syncer {
+
+// AttachmentDownloader is responsible for downloading attachments from server.
+class SYNC_EXPORT AttachmentDownloader {
+ public:
+  // The result of an DownloadAttachment operation.
+  enum DownloadResult {
+    DOWNLOAD_SUCCESS,            // No error, attachment was downloaded
+                                 // successfully.
+    DOWNLOAD_UNSPECIFIED_ERROR,  // An unspecified error occurred.
+  };
+
+  typedef base::Callback<void(const DownloadResult&, scoped_ptr<Attachment>)>
+      DownloadCallback;
+
+  virtual ~AttachmentDownloader();
+
+  // Download attachment referred by |attachment_id| and invoke |callback| when
+  // done.
+  //
+  // |callback| will receive an DownloadResult code and an Attachment object. If

[maniscalco, 2014/05/22 20:28:09]

nit: "an DownloadResult" -> "a DownloadResult"

[pavely, 2014/05/22 21:13:33]

Done.

+  // DownloadResult is not DOWNLOAD_SUCCESS then attachment pointer is NULL.
+  virtual void DownloadAttachment(const AttachmentId& attachment_id,
+                                  const DownloadCallback& callback) = 0;
+};
+
+}  // namespace syncer
+
+#endif  // SYNC_API_ATTACHMENTS_ATTACHMENT_DOWNLOADER_H_