[Offline Pages] Turn Offliner::Cancel into an async operation to resolve conflicting assumptions

components/offline_pages/core/background/offliner_stub.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/core/background/offliner_stub.h"
 
 #include "base/bind.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "components/offline_pages/core/background/save_page_request.h"
 
@@ skipping 16 matching lines @@
 
   // Post the callback on the run loop.
   if (enable_callback_) {
     base::ThreadTaskRunnerHandle::Get()->PostTask(
         FROM_HERE,
         base::Bind(callback, request, Offliner::RequestStatus::SAVED));
   }
   return true;
 }
 
-void OfflinerStub::Cancel() {
+void OfflinerStub::Cancel(const CancelCallback& callback) {
   cancel_called_ = true;
+  callback.Run(0LL);
 }
 
 bool OfflinerStub::HandleTimeout(const SavePageRequest& request) {
   if (snapshot_on_last_retry_) {
     base::ThreadTaskRunnerHandle::Get()->PostTask(
         FROM_HERE,
         base::Bind(callback_, request, Offliner::RequestStatus::SAVED));
     return true;
   }
   return false;
 }
 
 }  // namespace offline_pages