Fixes an invalidated pointer access in NTPSnipppetsFetcher.

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 0dc7634ef077b50b2a7d4cfd6f1b22ad1ed34acd..00166db78a618e17c4d311b6c3f251c5d8d76e85 100644
--- a/components/ntp_snippets/content_suggestions_provider.h
+++ b/components/ntp_snippets/content_suggestions_provider.h
@@ -109,7 +109,7 @@ class ContentSuggestionsProvider {
   // existing suggestions.
   virtual void Fetch(const Category& category,
                      const std::set<std::string>& known_suggestion_ids,
-                     FetchingCallback callback) = 0;
+                     const FetchingCallback& callback) = 0;
 
   // Removes history from the specified time range where the URL matches the
   // |filter|. The data removed depends on the provider. Note that the