Refactor OfflinePageSuggestionsProvider dismissed ID handling

components/ntp_snippets/content_suggestions_service.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_SERVICE_H_
 #define COMPONENTS_NTP_SNIPPETS_CONTENT_SUGGESTIONS_SERVICE_H_
 
 #include <map>
 #include <memory>
 #include <string>
@@ skipping 37 matching lines @@
     // displayed to the user. The UI must immediately clear any suggestions of
     // that category.
     virtual void OnCategoryStatusChanged(Category category,
                                          CategoryStatus new_status) = 0;
 
     // Fired when a suggestion has been invalidated. The UI must immediately
     // clear the suggestion even from open NTPs. Invalidation 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(Category category,
                                          const std::string& suggestion_id) = 0;
 
     // Sent when the service is shutting down. After the service has shut down,
     // it will not provide any data anymore, though calling the getters is still
     // safe.
     virtual void ContentSuggestionsServiceShutdown() = 0;
 
    protected:
     virtual ~Observer() {}
@@ skipping 161 matching lines @@
   // background fetching and debugging calls to it. If the NTPSnippetsService is
   // loaded, it is also present in |providers_|, otherwise this is a nullptr.
   NTPSnippetsService* ntp_snippets_service_;
 
   DISALLOW_COPY_AND_ASSIGN(ContentSuggestionsService);
 };
 
 }  // namespace ntp_snippets
 
 #endif  // COMPONENTS_NTP_SNIPPETS_CONTENT_SUGGESTIONS_SERVICE_H_