Add test of MakeUniqueID() and friends.

components/ntp_snippets/content_suggestions_provider.h

 // Copyright 2016 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_CONTENT_SUGGESTIONS_PROVIDER_H_
 #define COMPONENTS_NTP_SNIPPETS_CONTENT_SUGGESTIONS_PROVIDER_H_
 
 #include <string>
 #include <vector>
 
@@ skipping 115 matching lines @@
       const DismissedSuggestionsCallback& callback) = 0;
 
   // Used only for debugging purposes. Clears the cache of dismissed
   // suggestions for the given |category|, if present, so that no suggestions
   // are suppressed. This does not necessarily make previously dismissed
   // suggestions reappear, as they may have been permanently deleted, depending
   // on the provider implementation.
   virtual void ClearDismissedSuggestionsForDebugging(Category category) = 0;
 
  protected:
+  friend class ContentSuggestionsProviderTest;
+
   ContentSuggestionsProvider(Observer* observer,
                              CategoryFactory* category_factory);
 
   // Creates a unique ID. The given |within_category_id| must be unique among
   // all suggestion IDs from this provider for the given |category|. This method
   // combines it with the |category| to form an ID that is unique
   // application-wide, because this provider is the only one that provides
   // suggestions for that category.
   std::string MakeUniqueID(Category category,
                            const std::string& within_category_id) const;
 
   // Reverse functions for MakeUniqueID()
   Category GetCategoryFromUniqueID(const std::string& unique_id) const;
   std::string GetWithinCategoryIDFromUniqueID(
       const std::string& unique_id) const;
 
   Observer* observer() const { return observer_; }
   CategoryFactory* category_factory() const { return category_factory_; }
 
  private:
   Observer* observer_;
   CategoryFactory* category_factory_;
 };
 
 }  // namespace ntp_snippets
 
 #endif  // COMPONENTS_NTP_SNIPPETS_CONTENT_SUGGESTIONS_PROVIDER_H_