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

components/offline_pages/core/background/offliner.h

Index: components/offline_pages/core/background/offliner.h
diff --git a/components/offline_pages/core/background/offliner.h b/components/offline_pages/core/background/offliner.h
index a7a5718596cc1c89d7d29e26c13aa21730241d24..a018c18787d0c6a3a05bf6c2837ca3efd509becc 100644
--- a/components/offline_pages/core/background/offliner.h
+++ b/components/offline_pages/core/background/offliner.h
@@ -61,6 +61,9 @@ class Offliner {
   // 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() {}
@@ -73,7 +76,7 @@ class Offliner {
 
   // 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.
 };