[Sync] Add size and crc32c to AttachmentId.

sync/api/attachments/attachment_metadata.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_METADATA_H_
 #define SYNC_API_ATTACHMENTS_ATTACHMENT_METADATA_H_
 
 #include <vector>
 
 #include "base/basictypes.h"
@@ skipping 12 matching lines @@
 
   // Default copy and assignment are welcome.
 
   // Returns this attachment's id.
   const AttachmentId& GetId() const;
 
   // Returns this attachment's size in bytes.
   size_t GetSize() const;
 
  private:
+  // TODO(maniscalco): Reconcile AttachmentMetadata and
+  // AttachmentId. AttachmentId knows the size of the attachment so
+  // AttachmentMetadata may not be necessary (crbug/465375).
   AttachmentId id_;
   size_t size_;
 };
 
 typedef std::vector<syncer::AttachmentMetadata> AttachmentMetadataList;
 
 }  // namespace syncer
 
 #endif  // SYNC_API_ATTACHMENTS_ATTACHMENT_METADATA_H_