[Offline Pages] Adds storage for kInvalidOfflineId.

components/offline_pages/offline_page_model.cc

 // Copyright 2016 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/offline_pages/offline_page_model.h"
 
 #include "url/gurl.h"
 
 namespace offline_pages {
 
+const int64_t OfflinePageModel::kInvalidOfflineId;
+
 OfflinePageModel::SavePageParams::SavePageParams()
     : proposed_offline_id(OfflinePageModel::kInvalidOfflineId) {
 }
 
 OfflinePageModel::SavePageParams::SavePageParams(const SavePageParams& other) {
   url = other.url;
   client_id = other.client_id;
   proposed_offline_id = other.proposed_offline_id;
   original_url = other.original_url;
 }
 
 // static
 bool OfflinePageModel::CanSaveURL(const GURL& url) {
   return url.is_valid() && url.SchemeIsHTTPOrHTTPS();
 }
 
 OfflinePageModel::OfflinePageModel() {}
 
 OfflinePageModel::~OfflinePageModel() {}
 
 }  // namespace offline_pages