Implement an observer interface for the site engagement service.

chrome/browser/engagement/site_engagement_observer.h

Index: chrome/browser/engagement/site_engagement_observer.h
diff --git a/chrome/browser/engagement/site_engagement_observer.h b/chrome/browser/engagement/site_engagement_observer.h
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+++ b/chrome/browser/engagement/site_engagement_observer.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 CHROME_BROWSER_ENGAGEMENT_SITE_ENGAGEMENT_OBSERVER_H_
+#define CHROME_BROWSER_ENGAGEMENT_SITE_ENGAGEMENT_OBSERVER_H_
+
+#include "base/gtest_prod_util.h"
+#include "base/macros.h"
+
+class GURL;
+class SiteEngagementService;
+
+class SiteEngagementObserver {
+ public:
+  // Called when the engagement for |url| increases to |score|.
+  // This may be run on user input, so observers *must not* perform any
+  // expensive tasks in this callback.
+  virtual void OnEngagementIncreased(const SiteEngagementService* service,
+                                     const GURL& url,
+                                     double score) {}
+
+ protected:
+  explicit SiteEngagementObserver(SiteEngagementService* service);
+
+  SiteEngagementObserver();
+
+  ~SiteEngagementObserver();
+
+  // Begin observing |service| for engagement increases.
+  // To stop observing, call Observe(nullptr).
+  void Observe(SiteEngagementService* service);
+
+ private:
+  FRIEND_TEST_ALL_PREFIXES(SiteEngagementServiceTest, Observers);
+  SiteEngagementService* service_;
+
+  DISALLOW_COPY_AND_ASSIGN(SiteEngagementObserver);
+
+};
+
+#endif  // CHROME_BROWSER_ENGAGEMENT_SITE_ENGAGEMENT_OBSERVER_H_