Refactor and extend SnippetsBridge for multi-section support

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 52200626d34870bc2892cfde07b770e3894df07a..2f343a47dc4bf918334e9eea8d58026f078bf55b 100644
--- a/components/ntp_snippets/content_suggestions_provider.h
+++ b/components/ntp_snippets/content_suggestions_provider.h
@@ -90,7 +90,8 @@ 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.
+  // 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;