Make GetDismissedSuggestionsForDebugging asynchronous

components/ntp_snippets/offline_pages/offline_page_suggestions_provider.h

Index: components/ntp_snippets/offline_pages/offline_page_suggestions_provider.h
diff --git a/components/ntp_snippets/offline_pages/offline_page_suggestions_provider.h b/components/ntp_snippets/offline_pages/offline_page_suggestions_provider.h
index 35b6161385dcc72c1aef887cb2d605d98c772c21..3b3820c219f38bb10484aa2f3a54737f4a85193d 100644
--- a/components/ntp_snippets/offline_pages/offline_page_suggestions_provider.h
+++ b/components/ntp_snippets/offline_pages/offline_page_suggestions_provider.h
@@ -55,8 +55,9 @@ class OfflinePageSuggestionsProvider
   void FetchSuggestionImage(const std::string& suggestion_id,
                             const ImageFetchedCallback& callback) override;
   void ClearCachedSuggestionsForDebugging(Category category) override;
-  std::vector<ContentSuggestion> GetDismissedSuggestionsForDebugging(
-      Category category) override;
+  void GetDismissedSuggestionsForDebugging(
+      Category category,
+      const DismissedSuggestionsCallback& callback) override;
   void ClearDismissedSuggestionsForDebugging(Category category) override;
 
   // OfflinePageModel::Observer implementation.
@@ -71,6 +72,10 @@ class OfflinePageSuggestionsProvider
   // Callback from the OfflinePageModel.
   void OnOfflinePagesLoaded(
       const offline_pages::MultipleOfflinePageItemResult& result);
+  void OnOfflinePagesLoadedForDismissedDebugging(
+      Category category,
+      const DismissedSuggestionsCallback& callback,
+      const offline_pages::MultipleOfflinePageItemResult& result);
 
   // Updates the |category_status_| of the |category| and notifies the
   // |observer_|, if necessary.