[NTP Snippets] FetchMore backend

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 f6741356d66e6cfc0a3773c83c8afe25a6d63d83..7e5742e94fc6d4796c9f9b7c9249ff1c5f0918fb 100644
--- a/components/ntp_snippets/content_suggestions_provider.h
+++ b/components/ntp_snippets/content_suggestions_provider.h
@@ -31,6 +31,8 @@ class ContentSuggestionsProvider {
   using ImageFetchedCallback = base::Callback<void(const gfx::Image&)>;
   using DismissedSuggestionsCallback = base::Callback<void(
       std::vector<ContentSuggestion> dismissed_suggestions)>;
+  using FetchedMoreCallback =
+      base::Callback<void(std::vector<ContentSuggestion> suggestions)>;
 
   // The observer of a provider is notified when new data is available.
   class Observer {
@@ -100,6 +102,12 @@ class ContentSuggestionsProvider {
   virtual void FetchSuggestionImage(const ContentSuggestion::ID& suggestion_id,
                                     const ImageFetchedCallback& callback) = 0;
 
+  // A user-triggered request to fetch more content for the given category.
+  // Provides only suggestions that have not been provided. The given |callback|
+  // is called with these suggestion, along with all existing suggestions.
+  virtual void FetchMore(const Category& category,

[Bernhard Bauer, 2016/10/28 16:25:59]

If the plan is to migrate from the Observer to this method for regular fetches
as well, should we just from the start call this method just Fetch()?

[fhorschig, 2016/11/02 05:06:12]

Done.

+                         FetchedMoreCallback 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
   // data outside the time range may be deleted, for example suggestions, which