Change NTPSnippetsService to implement ContentSuggestionsProvider

components/ntp_snippets/content_suggestions_provider.h

Index: components/ntp_snippets/content_suggestions_provider.h
diff --git a/components/ntp_snippets/content_suggestions_provider.h b/components/ntp_snippets/content_suggestions_provider.h
index 93872f5843a8214d2b835e810e05c558c44f6435..8d0b5dd7e5d6c38ae55581b5ca84299aae71dac0 100644
--- a/components/ntp_snippets/content_suggestions_provider.h
+++ b/components/ntp_snippets/content_suggestions_provider.h
@@ -62,7 +62,8 @@ class ContentSuggestionsProvider {
   };
 
   // Sets an observer which is notified about changes to the available
-  // suggestions, or removes it by passing a nullptr.
+  // suggestions, or removes it by passing a nullptr. The provider does not take
+  // ownership of the observer and the observer must outlive this provider.
   virtual void SetObserver(Observer* observer) = 0;
 
   // Determines the status of the given |category|, see
@@ -78,6 +79,8 @@ class ContentSuggestionsProvider {
 
   // 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;