Refactor OfflinePageSuggestionsProvider dismissed ID handling

components/ntp_snippets/content_suggestions_provider.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_NTP_SNIPPETS_CONTENT_SUGGESTIONS_PROVIDER_H_
 #define COMPONENTS_NTP_SNIPPETS_CONTENT_SUGGESTIONS_PROVIDER_H_
 
 #include <string>
 #include <vector>
 
@@ skipping 54 matching lines @@
                                          Category category,
                                          CategoryStatus new_status) = 0;
 
     // Called when a suggestion has been invalidated. It will not be provided
     // through |OnNewSuggestions| anymore, is not supported by
     // |FetchSuggestionImage| or |DismissSuggestion| anymore, and should
     // immediately be cleared from the UI and caches. This happens, for example,
     // when the content that the suggestion refers to is gone.
     // Note that this event may be fired even if the corresponding |category| is
     // not currently AVAILABLE, because open UIs may still be showing the
-    // suggestion that is to be removed.
+    // suggestion that is to be removed. This event may also be fired for
+    // |suggestion_id|s that never existed and should be ignored in that case.
     virtual void OnSuggestionInvalidated(ContentSuggestionsProvider* provider,
                                          Category category,
                                          const std::string& suggestion_id) = 0;
   };
 
   virtual ~ContentSuggestionsProvider();
 
   // Returns the categories provided by this provider.
   // When the set of provided categories changes, the Observer is notified
   // through |OnNewSuggestions| or |OnCategoryStatusChanged| for added
@@ skipping 60 matching lines @@
   CategoryFactory* category_factory() const { return category_factory_; }
 
  private:
   Observer* observer_;
   CategoryFactory* category_factory_;
 };
 
 }  // namespace ntp_snippets
 
 #endif  // COMPONENTS_NTP_SNIPPETS_CONTENT_SUGGESTIONS_PROVIDER_H_