Add FetchMore to Content Suggestions Mediator

ios/chrome/browser/ui/content_suggestions/content_suggestions_data_source.h

 // Copyright 2017 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 IOS_CHROME_BROWSER_UI_CONTENT_SUGGESTIONS_CONTENT_SUGGESTIONS_DATA_SOURCE_H_
 #define IOS_CHROME_BROWSER_UI_CONTENT_SUGGESTIONS_CONTENT_SUGGESTIONS_DATA_SOURCE_H_
 
 @class ContentSuggestion;
+@class ContentSuggestionIdentifier;
 @class ContentSuggestionsSectionInformation;
 @protocol ContentSuggestionsDataSink;
 @protocol ContentSuggestionsImageFetcher;
 
+// Typedef for a block taking the fetched suggestions as parameter.
+typedef void (^MoreSuggestionsFetched)(NSArray<ContentSuggestion*>* _Nonnull);
+
 // DataSource for the content suggestions. Provides the suggestions data in a
 // format compatible with Objective-C.
 @protocol ContentSuggestionsDataSource
 
 // The data sink that will be notified when the data change.
 @property(nonatomic, nullable, weak) id<ContentSuggestionsDataSink> dataSink;
 
 // Returns all the data currently available.
 - (nonnull NSArray<ContentSuggestion*>*)allSuggestions;
 
 // Returns the data currently available for the section identified by
 // |sectionInfo|.
 - (nonnull NSArray<ContentSuggestion*>*)suggestionsForSection:
     (nonnull ContentSuggestionsSectionInformation*)sectionInfo;
 
 // Returns an image updater for the suggestions provided by this data source.
 - (nullable id<ContentSuggestionsImageFetcher>)imageFetcher;
 
+// Fetches additional content. All the |knownSuggestions| must come from the
+// same |sectionInfo|. If the fetch was completed, the given |callback| is
+// called with the new content.
+- (void)fetchMoreSuggestionsKnowing:
+            (nullable NSArray<ContentSuggestionIdentifier*>*)knownSuggestions
+                    fromSectionInfo:
+                        (nonnull ContentSuggestionsSectionInformation*)
+                            sectionInfo
+                           callback:(nullable MoreSuggestionsFetched)callback;
+
 @end
 
 #endif  // IOS_CHROME_BROWSER_UI_CONTENT_SUGGESTIONS_CONTENT_SUGGESTIONS_DATA_SOURCE_H_