Have the RequestCoordinator generate the offline_id

components/offline_pages/stub_offline_page_model.h

Index: components/offline_pages/stub_offline_page_model.h
diff --git a/components/offline_pages/stub_offline_page_model.h b/components/offline_pages/stub_offline_page_model.h
index 6d55280ea55414998bddda3114583a9ca9586ab4..d5e0fb80b31c1be0a2999ab587c0ac705f48924d 100644
--- a/components/offline_pages/stub_offline_page_model.h
+++ b/components/offline_pages/stub_offline_page_model.h
@@ -25,6 +25,7 @@ class StubOfflinePageModel : public OfflinePageModel {
   void RemoveObserver(Observer* observer) override;
   void SavePage(const GURL& url,
                 const ClientId& client_id,
+                int64_t proposed_offline_id,
                 std::unique_ptr<OfflinePageArchiver> archiver,
                 const SavePageCallback& callback) override;
   void MarkPageAccessed(int64_t offline_id) override;