Have the RequestCoordinator generate the offline_id

components/offline_pages/offline_page_model_impl.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_impl.h"
 
 #include <algorithm>
 #include <limits>
 
 #include "base/bind.h"
@@ skipping 256 matching lines @@
   observers_.AddObserver(observer);
 }
 
 void OfflinePageModelImpl::RemoveObserver(Observer* observer) {
   observers_.RemoveObserver(observer);
 }
 
 void OfflinePageModelImpl::SavePage(
     const GURL& url,
     const ClientId& client_id,
+    int64_t offline_id,
     std::unique_ptr<OfflinePageArchiver> archiver,
     const SavePageCallback& callback) {
   DCHECK(is_loaded_);
 
   // Skip saving the page that is not intended to be saved, like local file
   // page.
   if (!OfflinePageModel::CanSaveURL(url)) {
     InformSavePageDone(callback, SavePageResult::SKIPPED, client_id,
                        kInvalidOfflineId);
     return;
   }
 
   // The web contents is not available if archiver is not created and passed.
   if (!archiver.get()) {
     InformSavePageDone(callback, SavePageResult::CONTENT_UNAVAILABLE, client_id,
                        kInvalidOfflineId);
     return;
   }
 
-  int64_t offline_id = GenerateOfflineId();
+  // If we already have an offline id, use it.  If not, generate one.
+  if (offline_id == 0ul)

[Dmitry Titov, 2016/08/16 02:25:00]

This should go into the comment for SavePage method in .h file. This is
important part of the contract.

[Pete Williamson, 2016/08/16 18:54:56]

Done.

+    offline_id = GenerateOfflineId();
 
   archiver->CreateArchive(
       archives_dir_, offline_id,
       base::Bind(&OfflinePageModelImpl::OnCreateArchiveDone,
                  weak_ptr_factory_.GetWeakPtr(), url, offline_id, client_id,
                  base::Time::Now(), callback));
   pending_archivers_.push_back(std::move(archiver));
 }
 
 void OfflinePageModelImpl::MarkPageAccessed(int64_t offline_id) {
@@ skipping 750 matching lines @@
 void OfflinePageModelImpl::RunWhenLoaded(const base::Closure& task) {
   if (!is_loaded_) {
     delayed_tasks_.push_back(task);
     return;
   }
 
   base::ThreadTaskRunnerHandle::Get()->PostTask(FROM_HERE, task);
 }
 
 }  // namespace offline_pages