Implement an observer interface for the site engagement service.

chrome/browser/engagement/site_engagement_observer.cc

Index: chrome/browser/engagement/site_engagement_observer.cc
diff --git a/chrome/browser/engagement/site_engagement_observer.cc b/chrome/browser/engagement/site_engagement_observer.cc
new file mode 100644
index 0000000000000000000000000000000000000000..020ce792a579afd26c7ecea7fa266f9b37f57bda
--- /dev/null
+++ b/chrome/browser/engagement/site_engagement_observer.cc
@@ -0,0 +1,31 @@
+// 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.
+
+#include "chrome/browser/engagement/site_engagement_observer.h"
+
+#include "chrome/browser/engagement/site_engagement_service.h"
+
+SiteEngagementObserver::SiteEngagementObserver(SiteEngagementService* service)
+    : service_(nullptr) {
+  Observe(service);
+}
+
+SiteEngagementObserver::SiteEngagementObserver() : service_(nullptr) {}
+
+SiteEngagementObserver::~SiteEngagementObserver() {
+  if (service_)
+    service_->RemoveObserver(this);
+}
+
+void SiteEngagementObserver::Observe(SiteEngagementService* service) {
+  if (service == service_)
+    return;
+
+  if (service_)
+    service_->RemoveObserver(this);
+
+  service_ = service;
+  if (service_)
+    service->AddObserver(this);
+}