Add component for feature engagement tracking.

components/feature_engagement_tracker/internal/feature_engagement_tracker_impl.h

Index: components/feature_engagement_tracker/internal/feature_engagement_tracker_impl.h
diff --git a/components/feature_engagement_tracker/internal/feature_engagement_tracker_impl.h b/components/feature_engagement_tracker/internal/feature_engagement_tracker_impl.h
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+// Copyright 2017 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_FEATURE_ENGAGEMENT_TRACKER_INTERNAL_FEATURE_ENGAGEMENT_TRACKER_IMPL_H_
+#define COMPONENTS_FEATURE_ENGAGEMENT_TRACKER_INTERNAL_FEATURE_ENGAGEMENT_TRACKER_IMPL_H_
+
+#include <string>
+
+#include "base/macros.h"
+#include "base/supports_user_data.h"
+#include "components/feature_engagement_tracker/public/feature_engagement_tracker.h"
+
+namespace feature_engagement_tracker {
+
+// The internal implementation of the FeatureEngagementTracker.
+class FeatureEngagementTrackerImpl : public FeatureEngagementTracker,
+                                     public base::SupportsUserData {
+ public:
+  FeatureEngagementTrackerImpl();
+  ~FeatureEngagementTrackerImpl() override;
+
+  // FeatureEngagementTracker implementation.
+  void Event(const std::string& feature,
+             const std::string& precondition) override;
+  void Used(const std::string& feature) override;
+  bool Trigger(const std::string& feature) override;
+  void Dismissed() override;
+  void AddOnInitializedCallback(OnInitializedCallback callback) override;
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(FeatureEngagementTrackerImpl);
+};
+
+}  // namespace feature_engagement_tracker
+
+#endif  // COMPONENTS_FEATURE_ENGAGEMENT_TRACKER_INTERNAL_FEATURE_ENGAGEMENT_TRACKER_IMPL_H_