components: Add out-of-line copy ctors for complex classes.

components/history/core/browser/download_row.cc

 // Copyright (c) 2011 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 "components/history/core/browser/download_row.h"
 
 #include "components/history/core/browser/download_constants.h"
 
 namespace history {
 
@@ skipping 40 matching lines @@
       received_bytes(received),
       total_bytes(total),
       state(download_state),
       danger_type(danger_type),
       interrupt_reason(interrupt_reason),
       id(id),
       opened(download_opened),
       by_ext_id(ext_id),
       by_ext_name(ext_name) {}
 
+DownloadRow::DownloadRow(const DownloadRow& other) = default;
+
 DownloadRow::~DownloadRow() {
 }
 
 }  // namespace history