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

components/ntp_snippets/category_rankers/category_ranker.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_RANKERS_CATEGORY_RANKER_H_
+#define COMPONENTS_NTP_SNIPPETS_CATEGORY_RANKERS_CATEGORY_RANKER_H_
+
+#include "base/time/time.h"
+#include "components/ntp_snippets/category.h"
+
+namespace ntp_snippets {
+
+// TODO(vitaliii): Ensure that changes in the order are propagated to the UI.
+// (crbug.com/673743)
+
+// Orders sections (represented as categories).
+// The order may be dynamic and change at any time.
+class CategoryRanker {
+ public:
+  // Compares two categories. True means that |left| precedes |right|, i.e. it
+  // must be located earlier (above) on the NTP. This method must satisfy
+  // "Compare" contract required by sort algorithms.
+  virtual bool Compare(Category left, Category right) const = 0;
+
+  // Deletes all history related data between |begin| and |end|. After this
+  // call, the category order may not depend on this history range anymore.
+  virtual void ClearHistory(base::Time begin, base::Time end) = 0;

[tschumann, 2016/12/15 11:50:26]

is this actually a necessary requirement? Would be good to double-check  so that
we don't make our lives unnecessarily hard.
Given you already did the work, we can move ahead as is but getting clarity
would be nice (maybe you already have ;-)).

[vitaliii, 2016/12/15 15:30:13]

My reasoning for this was that we had to provide such method for suggestion
providers.
Overall, this is an internal interface with at most 3 implementations in the
nearest future, so it should be ok to change it accordingly if anything changes.

[tschumann, 2016/12/15 18:23:13]

What I actually mean, is are we required to delete the category ordering when
the user clears their history.
If not , then we should even bother about this.

[vitaliii, 2016/12/16 08:15:43]

We are not required to do this (yet), however, I feel like we will and,
moreover, I was planning to add this function to snippets_internals. 
I am fine to remove it later if it happens to be not used.

+
+  // If |category| has not been added previously, it is added after all already
+  // known categories, otherwise nothing is changed.
+  virtual void AppendCategoryIfNecessary(Category category) = 0;
+
+  // Feedback data from the user to update the ranking.
+
+  // Called whenever a suggestion is opened by the user.
+  virtual void OnSuggestionOpened(Category category) = 0;
+};
+
+}  // namespace ntp_snippets
+
+#endif  // COMPONENTS_NTP_SNIPPETS_CATEGORY_RANKERS_CATEGORY_RANKER_H_