Add component for feature engagement tracking.

components/feature_engagement_tracker/public/android/java/src/org/chromium/components/feature_engagement_tracker/FeatureEngagementTracker.java

+// 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.
+
+package org.chromium.components.feature_engagement_tracker;
+
+import android.support.annotation.CheckResult;
+
+import org.chromium.base.Callback;
+
+/**
+ * FeatureEngagementTracker is the Java representation of a native FeatureEngagementTracker object.
+ * It is owned by the native BrowserContext.
+ *
+ * FeatureEngagementTracker is the core class for the feature engagement tracker.
+ */
+public interface FeatureEngagementTracker {
+    /**
+     * Must be called whenever an event related to a precondition happens.
+     */
+    void event(String feature, String precondition);
+
+    /**
+     * Must be called whenever a feature has been used.
+     */
+    void used(String feature);
+
+    /**
+     * This function must be called whenever the triggering condition for a
+     * specific feature happens. Returns true iff the display of feature
+     * enlightenment must happen.
+     * If {@code true} is returned, the caller *must* call {@link #dismissed()} when display
+     * of feature enlightenment ends.
+     * @return whether feature enlightenment should be displayed.
+     */
+    @CheckResult
+    boolean trigger(String feature);
+
+    /**
+     * Must be called after display of feature enlightenment finishes.
+     */
+    void dismissed();
+
+    /**
+     * For features that trigger on startup, they register a callback to ensure that they are told
+     * when the tracker has been initialized. The callback will be invoked when the tracker has
+     * been initialized. The boolean parameter indicated whether the initialization was a success
+     * and that the tracker is ready to receive calls.
+     */
+    void addOnInitializedCallback(Callback<Boolean> callback);
+}