Grant origins engagement for having interactions on their notifications.

chrome/browser/engagement/site_engagement_observer.h

 // 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"
 
 namespace content {
 class WebContents;
 }
 
 class GURL;
 class SiteEngagementService;
 
 class SiteEngagementObserver {
  public:
   // Called when the engagement for |url| loaded in |web_contents| increases to
   // |score|. |is_hidden| is true if the engagement occurred when |web_contents|
-  // was hidden (e.g. in the background).
-  // This method may be run on user input, so observers *must not* perform any
-  // expensive tasks here.
+  // was hidden (e.g. in the background). This method may be run on user input,
+  // so observers *must not* perform any expensive tasks here. |web_contents|
+  // may be null if the engagement has increased when |url| is not in a tab,
+  // e.g. from a notification interaction.
   virtual void OnEngagementIncreased(content::WebContents* web_contents,
                                      const GURL& url,
                                      double score) {}
 
  protected:
   explicit SiteEngagementObserver(SiteEngagementService* service);
 
   SiteEngagementObserver();
 
   ~SiteEngagementObserver();
 
   // Returns the site engagement service which this object is observing.
   SiteEngagementService* GetSiteEngagementService() const;
 
   // 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_