Enable SiteEngagementEvictionPolicy behind --enable-site-eviction-policy flag.

chrome/browser/engagement/site_engagement_service.h

 // Copyright 2015 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_SERVICE_H_
 #define CHROME_BROWSER_ENGAGEMENT_SITE_ENGAGEMENT_SERVICE_H_
 
 #include <map>
 #include <set>
 
@@ skipping 105 matching lines @@
 // engaged with an origin - the higher it is, the more engagement the user has
 // had with this site recently.
 //
 // Positive user activity, such as visiting the origin often and adding it to
 // the homescreen, will increase the site engagement score. Negative activity,
 // such as rejecting permission prompts or not responding to notifications, will
 // decrease the site engagement score.
 class SiteEngagementService : public KeyedService,
                               public SiteEngagementScoreProvider {
  public:
+  // The name of the site engagement variation field trial.
+  static const char kEngagementParams[];
+
   static SiteEngagementService* Get(Profile* profile);
 
   // Returns whether or not the SiteEngagementService is enabled.
   static bool IsEnabled();
 
   // Clears engagement scores for the given origins.
   static void ClearHistoryForURLs(Profile* profile,
                                   const std::set<GURL>& origins);
 
   explicit SiteEngagementService(Profile* profile);
@@ skipping 54 matching lines @@
   // origin's engagement score after an hour has elapsed triggers the next
   // upload.
   base::Time last_metrics_time_;
 
   base::WeakPtrFactory<SiteEngagementService> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(SiteEngagementService);
 };
 
 #endif  // CHROME_BROWSER_ENGAGEMENT_SITE_ENGAGEMENT_SERVICE_H_