[Offline pages] Introduction of MarkAttemptStartedTask with tests

components/offline_pages/background/request_coordinator.h

Index: components/offline_pages/background/request_coordinator.h
diff --git a/components/offline_pages/background/request_coordinator.h b/components/offline_pages/background/request_coordinator.h
index 371fd23c319dac2200575e2fdbc700c98867a499..780fd4032f763ea0a87b02404aacbdd75dcda94d 100644
--- a/components/offline_pages/background/request_coordinator.h
+++ b/components/offline_pages/background/request_coordinator.h
@@ -283,8 +283,15 @@ class RequestCoordinator : public KeyedService,
   // Cancels an in progress pre-rendering, and updates state appropriately.
   void StopPrerendering(Offliner::RequestStatus stop_status);
 
+  // Marks attempt on the request and sends it to offliner in continuation.
   void SendRequestToOffliner(const SavePageRequest& request);
 
+  // Continuation of |SendRequestToOffliner| after the request is marked as
+  // started.
+  void StartOffliner(int64_t request_id,
+                     const std::string& client_namespace,
+                     std::unique_ptr<UpdateRequestsResult> update_result);
+
   // Called by the offliner when an offlining request is completed. (and by
   // tests).
   void OfflinerDoneCallback(const SavePageRequest& request,