Change NTPSnippetsService to implement ContentSuggestionsProvider

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 60 matching lines @@
       ContentSuggestionsCategory category) = 0;
 
   // Discards the suggestion with the given ID. A provider needs to ensure that
   // a once-discarded suggestion is never delivered again (through the
   // Observer). The provider must not call Observer::OnSuggestionsChanged if the
   // removal of the discarded suggestion is the only change.
   virtual void DiscardSuggestion(const std::string& suggestion_id) = 0;
 
   // Fetches the image for the suggestion with the given ID and returns it
   // through the callback. This fetch may occur locally or from the internet.
+  // If that suggestion doesn't exist, doesn't have an image or if the fetch
+  // fails, the callback gets a null image.
   virtual void FetchSuggestionImage(const std::string& suggestion_id,
                                     const ImageFetchedCallback& callback) = 0;
 
   // Used only for debugging purposes. Clears all caches so that the next
   // fetch starts from scratch.
   virtual void ClearCachedSuggestionsForDebugging() = 0;
 
   // Used only for debugging purposes. Clears the cache of discarded
   // suggestions, if present, so that no suggestions are suppressed. This does
   // not necessarily make previously discarded suggestions reappear, as they may
@@ skipping 17 matching lines @@
   static std::string MakeUniqueID(ContentSuggestionsCategory category,
                                   const std::string& within_category_id);
 
  private:
   const std::vector<ContentSuggestionsCategory> provided_categories_;
 };
 
 }  // namespace ntp_snippets
 
 #endif  // COMPONENTS_NTP_SNIPPETS_CONTENT_SUGGESTIONS_PROVIDER_H_