Add LanguageModel, a keyed service that collects language info from CLD.

components/translate/core/browser/language_model.h

Index: components/translate/core/browser/language_model.h
diff --git a/components/translate/core/browser/language_model.h b/components/translate/core/browser/language_model.h
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index 0000000000000000000000000000000000000000..bf6d2225311cb43e03182182c56787a1967c1877
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+++ b/components/translate/core/browser/language_model.h
@@ -0,0 +1,61 @@
+// 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_TRANSLATE_CORE_BROWSER_LANGUAGE_MODEL_H_
+#define COMPONENTS_TRANSLATE_CORE_BROWSER_LANGUAGE_MODEL_H_
+
+#include <string>
+#include <vector>
+
+#include "base/macros.h"
+#include "components/keyed_service/core/keyed_service.h"
+
+class PrefRegistrySimple;
+class PrefService;
+
+namespace translate {
+
+// Collects data about languages in which the user reads the web and provides
+// access to current estimated language preferences. The past behaviour is
+// discounted so that this model reflects changes in browsing habits. This model
+// does not have to contain all languages that ever appeared in user's browsing,
+// languages with insignificant frequency are removed, eventually.
+class LanguageModel : public KeyedService {
+ public:
+  struct LanguageInfo {
+    // The ISO 639 language code.
+    std::string language_code;
+    // The current estimated frequency of the language share, a number between 0

[Bernhard Bauer, 2016/10/05 14:48:16]

Nit: empty line before blocks of comments please, for readability (also below).

[jkrcal, 2016/10/05 15:14:47]

Done.

+    // and 1 (can be understood as the probability that the next page the user
+    // opens is in this language).

[Bernhard Bauer, 2016/10/05 14:48:16]

Does that mean that the frequencies should sum to 1?

[jkrcal, 2016/10/05 15:14:46]

Yes, added a comment. The sum is not guaranteed to be exactly one (rounding
errors) but I think this is not worth stressing.

+    float frequency;
+
+    bool operator==(const LanguageInfo& m) const {
+      return language_code == m.language_code && frequency == m.frequency;
+    }
+  };
+
+  explicit LanguageModel(PrefService* pref_service);
+  ~LanguageModel() override;
+
+  // Registers profile prefs for the model.
+  static void RegisterProfilePrefs(PrefRegistrySimple* registry);
+
+  // Returns a list of the languages currently tracked by the model, sorted by
+  // frequency in decreasing order.
+  std::vector<LanguageInfo> GetTopLanguages() const;
+  // Returns the estimated frequency for the given language or 0 if the language
+  // is not among the top languages kept in the model.
+  float GetLanguageFrequency(const std::string& language_code) const;
+
+  // Informs the model that a page with the given language has been visited.
+  void OnPageVisited(const std::string& language_code);
+
+ private:
+  PrefService* pref_service_;
+};
+
+}  // namespace translate
+
+#endif  // COMPONENTS_TRANSLATE_CORE_BROWSER_LANGUAGE_MODEL_H_