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

components/offline_pages/core/background/offliner_stub.h

 // 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.
 
 #ifndef COMPONENTS_OFFLINE_PAGES_CORE_BACKGROUND_OFFLINER_STUB_H_
 #define COMPONENTS_OFFLINE_PAGES_CORE_BACKGROUND_OFFLINER_STUB_H_
 
 #include "components/offline_pages/core/background/offliner.h"
 
 namespace offline_pages {
 
 // Test class stubbing out the functionality of Offliner.
 // It is only used for test support.
 class OfflinerStub : public Offliner {
  public:
   OfflinerStub();
   ~OfflinerStub() override;
 
   bool LoadAndSave(const SavePageRequest& request,
                    const CompletionCallback& callback) override;
 
-  void Cancel() override;
+  void Cancel(const CancelCallback& callback) override;
 
   void disable_loading() { disable_loading_ = true; }
 
   void enable_callback(bool enable) { enable_callback_ = enable; }
 
   bool cancel_called() { return cancel_called_; }
 
   void reset_cancel_called() { cancel_called_ = false; }
 
   bool HandleTimeout(const SavePageRequest& request) override;
 
   void enable_snapshot_on_last_retry() { snapshot_on_last_retry_ = true; }
 
  private:
   CompletionCallback callback_;
   bool disable_loading_;
   bool enable_callback_;
   bool cancel_called_;
   bool snapshot_on_last_retry_;
 };
 
 }  // namespace offline_pages
 
 #endif  // COMPONENTS_OFFLINE_PAGES_CORE_BACKGROUND_OFFLINER_STUB_H_