Checkpointing some Sync Attachment work. Added SyncAttachment and SyncAttachmentId.

sync/api/sync_attachment.h

+// Copyright 2013 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_SYNC_ATTACHMENT_H_
+#define SYNC_API_SYNC_ATTACHMENT_H_
+
+#include "base/basictypes.h"
+#include "base/memory/ref_counted.h"
+#include "base/memory/scoped_ptr.h"
+#include "sync/base/sync_export.h"
+#include "sync/protocol/sync.pb.h"
+
+namespace base {
+class RefCountedMemory;
+}  // namespace base
+
+namespace syncer {
+
+// An immutable blob of in-memory data attached to a sync item.
+class SYNC_EXPORT SyncAttachment {
+ public:
+  ~SyncAttachment();
+
+  // Creates an attachment with a unique id and the supplied bytes.
+  //
+  // Used when creating a brand new attachment.
+  static scoped_ptr<SyncAttachment> Create(
+      const scoped_refptr<base::RefCountedMemory>& bytes);
+
+  // Creates an attachment with the supplied id and bytes.
+  //
+  // Used when you want to recreate a specific attachment. E.g. creating a local
+  // copy of an attachment that already exists on the sync server.
+  static scoped_ptr<SyncAttachment> CreateWithId(
+      const sync_pb::SyncAttachmentId& id,
+      const scoped_refptr<base::RefCountedMemory>& bytes);
+
+  // Returns this attachment's id.
+  const sync_pb::SyncAttachmentId& GetId() const;
+
+  // Returns this attachment's bytes.
+  const scoped_refptr<base::RefCountedMemory>& GetBytes() const;
+
+ private:
+  const sync_pb::SyncAttachmentId id_;
+  const scoped_refptr<base::RefCountedMemory> bytes_;
+
+  SyncAttachment(const sync_pb::SyncAttachmentId& id,
+                 const scoped_refptr<base::RefCountedMemory>& bytes);
+
+  // Default copy ctor welcome.
+  DISALLOW_ASSIGN(SyncAttachment);
+};
+
+}  // namespace syncer
+
+#endif  // SYNC_API_SYNC_ATTACHMENT_H_