FetchMore functionality 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|

[Marc Treib, 2016/10/20 16:51:39]

What if I call this method twice in a row - will I get the same suggestions
twice? If not, how does the provider know what to provide?

Example: User is on the NTP, sees 10 suggestions. Clicks more -> 10 more (call
them #11-#20). Clicks more again -> another 10 (#21-#30).
However, now the user opens a new NTP, and gets the first 10 suggestions again.
Clicks more -> we'd like to return #11-#20 again.
I think it might be necessary to pass in a list of "known" suggestions. WDYT?

[Marc Treib, 2016/10/28 14:49:49]

Also discussed with dgn and bauerb: Yep, we'll pass a list of known suggestion
IDs.

[fhorschig, 2016/11/02 05:05:26]

Done in CL 2466863003.

+  // is called with these suggestion, along with all existing suggestions.
+  virtual void FetchMore(const Category& category,
+                         FetchedMoreCallback callback) = 0;

[Marc Treib, 2016/10/20 16:51:39]

This should be a const&.
(Or alternatively, it could be a OnceCallback, but then we should also change
that in other places, like ImageFetchedCallback above. Probably best to do that
separately.)

[fhorschig, 2016/11/02 05:05:27]

Done. It's const for now.

+
   // 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