Implement ClearHistory for all ntp_snippets providers

components/ntp_snippets/ntp_snippets_service.h

 // Copyright 2015 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 COMPONENTS_NTP_SNIPPETS_NTP_SNIPPETS_SERVICE_H_
 #define COMPONENTS_NTP_SNIPPETS_NTP_SNIPPETS_SERVICE_H_
 
 #include <stddef.h>
 
 #include <memory>
@@ skipping 143 matching lines @@
   std::set<std::string> GetSuggestionsHosts() const;
 
   // Returns the maximum number of snippets that will be shown at once.
   static int GetMaxSnippetCountForTesting();
 
   // Available snippets, only for unit tests.
   const NTPSnippet::PtrVector& GetSnippetsForTesting() const {
     return snippets_;
   }
 
+  // Dismissed snippets, only for unit tests.
+  const NTPSnippet::PtrVector& GetDismissedSnippetsForTesting() const {
+    return dismissed_snippets_;
+  }
+
  private:
   friend class NTPSnippetsServiceTest;
   FRIEND_TEST_ALL_PREFIXES(NTPSnippetsServiceTest, StatusChanges);
 
   // Possible state transitions:
   //       NOT_INITED --------+
   //       /       \          |
   //      v         v         |
   //   READY <--> DISABLED    |
   //       \       /          |
@@ skipping 172 matching lines @@
 
   // Request throttler for limiting requests to thumbnail images.
   RequestThrottler thumbnail_requests_throttler_;
 
   DISALLOW_COPY_AND_ASSIGN(NTPSnippetsService);
 };
 
 }  // namespace ntp_snippets
 
 #endif  // COMPONENTS_NTP_SNIPPETS_NTP_SNIPPETS_SERVICE_H_