Implement ClearHistory for all ntp_snippets providers

components/ntp_snippets/ntp_snippets_service.h

Index: components/ntp_snippets/ntp_snippets_service.h
diff --git a/components/ntp_snippets/ntp_snippets_service.h b/components/ntp_snippets/ntp_snippets_service.h
index b8e1d930d27c903a6b11a14b991a4e6401d15f20..fce675592f628e8aa5f41a6d33c013af5a7bbc83 100644
--- a/components/ntp_snippets/ntp_snippets_service.h
+++ b/components/ntp_snippets/ntp_snippets_service.h
@@ -161,6 +161,11 @@ class NTPSnippetsService : public ContentSuggestionsProvider,
     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);