[Sync] Removing duplicated includes between cc and h files.

components/sync/api/attachments/attachment_metadata.cc

 // 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.
 
-#include <stddef.h>
-
 #include "components/sync/api/attachments/attachment_metadata.h"
 
 namespace syncer {
 
 AttachmentMetadata::AttachmentMetadata(const AttachmentId& id, size_t size)
     : id_(id), size_(size) {}
 
 AttachmentMetadata::~AttachmentMetadata() {}
 
 const AttachmentId& AttachmentMetadata::GetId() const {
   return id_;
 }
 
 size_t AttachmentMetadata::GetSize() const {
   return size_;
 }
 
 }  // namespace syncer