[NTP::PhysicalWeb] Implement suggestion dismissal.

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 <set>
@@ skipping 104 matching lines @@
       Category category) const;
 
   // Fetches the image for the suggestion with the given |suggestion_id| and
   // runs the |callback|. If that suggestion doesn't exist or the fetch fails,
   // the callback gets an empty image. The callback will not be called
   // synchronously.
   void FetchSuggestionImage(const ContentSuggestion::ID& suggestion_id,
                             const ImageFetchedCallback& callback);
 
   // Dismisses the suggestion with the given |suggestion_id|, if it exists.
-  // This will not trigger an update through the observers.
+  // This will not trigger an update through the observers (i.e. providers must
+  // not call |Observer::OnNewSuggestions|).
   void DismissSuggestion(const ContentSuggestion::ID& suggestion_id);
 
   // Dismisses the given |category|, if it exists.
   // This will not trigger an update through the observers.
   void DismissCategory(Category category);
 
   // Restores all dismissed categories.
   // This will not trigger an update through the observers.
   void RestoreDismissedCategories();
 
@@ skipping 184 matching lines @@
   PrefService* pref_service_;
 
   UserClassifier user_classifier_;
 
   DISALLOW_COPY_AND_ASSIGN(ContentSuggestionsService);
 };
 
 }  // namespace ntp_snippets
 
 #endif  // COMPONENTS_NTP_SNIPPETS_CONTENT_SUGGESTIONS_SERVICE_H_