[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 b03fe85ded0986ea96e360dc5b32110acdc892bc..1cb5a9c06fcdcd7662212620b137f5ca39a2b224 100644
--- a/components/offline_pages/background/request_coordinator.h
+++ b/components/offline_pages/background/request_coordinator.h
@@ -265,8 +265,15 @@ 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);

[Pete Williamson, 2016/10/14 17:02:35]

Should this be another task to both mark started and send to offliner? 
(Apologies if that is part of the design, and you just haven't coded it yet).

[dougarnett, 2016/10/14 20:12:31]

But I don't think this does any other queue/store operations besides MarkStarted
so seems good to just have the queue Task cover the db transaction-ish scope but
not more.

[fgorski, 2016/10/14 20:37:19]

No, because right now we are not planning to serialize everything the request
coordinator is doing, only the database access (through request queue). If as a
result of offlining we are getting into situation where something needs to be
written, will will call request queue from there to schedule a relevant task.
And I think Doug provides similar explanation here.

 
+  // 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,