Make ContentSuggestionsService recognize new/removed categories

components/ntp_snippets/category.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_CATEGORY_H_
 #define COMPONENTS_NTP_SNIPPETS_CATEGORY_H_
 
 #include <ostream>
 
 namespace ntp_snippets {
@@ skipping 22 matching lines @@
   // |CategoryFactory::FromIDValue(id)|.
   // Note that these IDs are not necessarily stable across multiple runs of
   // the application, so they should not be persisted.
   int id() const { return id_; }
 
   bool IsKnownCategory(KnownCategories known_category) const;
 
  private:
   friend class CategoryFactory;
 
-  Category(int id);
+  explicit Category(int id);
 
   int id_;
 
   // Allow copy and assignment.
 };
 
 bool operator==(const Category& left, const Category& right);
 
 bool operator!=(const Category& left, const Category& right);
 
 std::ostream& operator<<(std::ostream& os, const Category& obj);
 
 }  // namespace ntp_snippets
 
 #endif  // COMPONENTS_NTP_SNIPPETS_CATEGORY_H_