[NTP::SectionOrder] Replace CategoryFactory with a category ranker.

components/ntp_snippets/category_rankers/constant_category_ranker.h

Index: components/ntp_snippets/category_rankers/constant_category_ranker.h
diff --git a/components/ntp_snippets/category_rankers/constant_category_ranker.h b/components/ntp_snippets/category_rankers/constant_category_ranker.h
new file mode 100644
index 0000000000000000000000000000000000000000..c01f0b969dbba70acc56c7f763106fcfe9c7fac6
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+++ b/components/ntp_snippets/category_rankers/constant_category_ranker.h
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+// 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_SECTION_RANKERS_CONSTANT_SECTION_RANKER_H_
+#define COMPONENTS_NTP_SNIPPETS_SECTION_RANKERS_CONSTANT_SECTION_RANKER_H_
+
+#include <vector>
+
+#include "base/time/time.h"
+#include "components/ntp_snippets/category.h"
+#include "components/ntp_snippets/category_rankers/category_ranker.h"
+
+namespace ntp_snippets {
+
+// Simple implementation of a CategoryRanker that never changes the order. For
+// KnownCategories, the order is hardcoded. Remote categories must be added
+// using |AppendCategoryIfNecessary|. They are sorted after all known
+// categories. Among themselves their order is the same as the order they were
+// added in.
+class ConstantCategoryRanker : public CategoryRanker {
+ public:
+  ConstantCategoryRanker();
+  ~ConstantCategoryRanker() override;
+
+  // CategoryRanker implementation.
+  bool Compare(Category left, Category right) const override;
+  void ClearHistory(base::Time begin, base::Time end) override;
+  void AppendCategoryIfNecessary(Category category) override;
+  void OnSuggestionOpened(Category category) override;
+
+ private:
+  void AppendKnownCategory(KnownCategories known_category);
+
+  std::vector<Category> ordered_categories_;
+
+  DISALLOW_COPY_AND_ASSIGN(ConstantCategoryRanker);
+};
+
+}  // namespace ntp_snippets
+
+#endif  // COMPONENTS_NTP_SNIPPETS_SECTION_RANKERS_CONSTANT_SECTION_RANKER_H_