Have the RequestCoordinator generate the offline_id

components/offline_pages/stub_offline_page_model.cc

Index: components/offline_pages/stub_offline_page_model.cc
diff --git a/components/offline_pages/stub_offline_page_model.cc b/components/offline_pages/stub_offline_page_model.cc
index 68119ad8bd34a7d5c2fe6764d40296c86a894336..8c33134f52219fdc1b23f244b29f85ed7b780ff6 100644
--- a/components/offline_pages/stub_offline_page_model.cc
+++ b/components/offline_pages/stub_offline_page_model.cc
@@ -14,6 +14,7 @@ void StubOfflinePageModel::RemoveObserver(Observer* observer) {}
 void StubOfflinePageModel::SavePage(
     const GURL& url,
     const ClientId& client_id,
+    int64_t proposed_offline_id,
     std::unique_ptr<OfflinePageArchiver> archiver,
     const SavePageCallback& callback) {}
 void StubOfflinePageModel::MarkPageAccessed(int64_t offline_id) {}