Refactor and extend SnippetsBridge for multi-section support

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 72 matching lines @@
 
   // Dismisses the suggestion with the given ID. A provider needs to ensure that
   // a once-dismissed suggestion is never delivered again (through the
   // Observer). The provider must not call Observer::OnSuggestionsChanged if the
   // removal of the dismissed suggestion is the only change.
   virtual void DismissSuggestion(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.
+  // fails, the callback gets a null image. The callback will not be called
+  // synchronously.
   virtual void FetchSuggestionImage(const std::string& suggestion_id,
                                     const ImageFetchedCallback& callback) = 0;
 
   // Used only for debugging purposes. Clears all caches for the given category,
   // so that the next fetch starts from scratch.
   virtual void ClearCachedSuggestionsForDebugging(Category category) = 0;
 
   // Used only for debugging purposes. Retrieves suggestions for the given
   // |category| that have previously been dismissed and are still stored in the
   // provider. If the provider doesn't store dismissed suggestions for the given
@@ skipping 28 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_