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

components/offline_pages/core/background/offliner.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_H_
 #define COMPONENTS_OFFLINE_PAGES_CORE_BACKGROUND_OFFLINER_H_
 
 #include <string>
 
 #include "base/callback.h"
@@ skipping 43 matching lines @@
     // updating the status in the request queue failed.
     QUEUE_UPDATE_FAILED = 13,
     // NOTE: insert new values above this line and update histogram enum too.
     STATUS_COUNT
   };
 
   // Reports the completion status of a request.
   // TODO(dougarnett): consider passing back a request id instead of request.
   typedef base::Callback<void(const SavePageRequest&, RequestStatus)>
       CompletionCallback;
+  // Reports that the cancel operation has completed.
+  // TODO(chili): make save operation cancellable.
+  typedef base::Callback<void(int64_t request_id)> CancelCallback;
 
   Offliner() {}
   virtual ~Offliner() {}
 
   // Processes |request| to load and save an offline page.
   // Returns whether the request was accepted or not. |callback| is guaranteed
   // to be called if the request was accepted and |Cancel()| is not called.
   virtual bool LoadAndSave(const SavePageRequest& request,
                            const CompletionCallback& callback) = 0;
 
   // Clears the currently processing request, if any, and skips running its
   // CompletionCallback.
-  virtual void Cancel() = 0;
+  virtual void Cancel(const CancelCallback& callback) = 0;
 
   // TODO(dougarnett): add policy support methods.
 };
 
 }  // namespace offline_pages
 
 #endif  // COMPONENTS_OFFLINE_PAGES_CORE_BACKGROUND_OFFLINER_H_