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

chrome/browser/engagement/site_engagement_eviction_policy.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_EVICTION_POLICY_H_
 #define CHROME_BROWSER_ENGAGEMENT_SITE_ENGAGEMENT_EVICTION_POLICY_H_
 
 #include <map>
 
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/scoped_ptr.h"
 #include "storage/browser/quota/quota_manager.h"
 #include "url/gurl.h"
 
 namespace content {
 class BrowserContext;
 }
 
 class SiteEngagementScoreProvider;
 
 class SiteEngagementEvictionPolicy : public storage::QuotaEvictionPolicy {
  public:
+  static bool IsEnabled();
+
   explicit SiteEngagementEvictionPolicy(
       content::BrowserContext* browser_context);
   ~SiteEngagementEvictionPolicy() override;
 
   // Overridden from storage::QuotaEvictionPolicy:
   void GetEvictionOrigin(const scoped_refptr<storage::SpecialStoragePolicy>&
                              special_storage_policy,
                          const std::set<GURL>& exceptions,
                          const std::map<GURL, int64>& usage_map,
                          int64 global_quota,
                          const storage::GetOriginCallback& callback) override;
 
  private:
   friend class SiteEngagementEvictionPolicyTest;
 
   static GURL CalculateEvictionOriginForTests(
       const scoped_refptr<storage::SpecialStoragePolicy>&
           special_storage_policy,
       SiteEngagementScoreProvider* score_provider,
       const std::set<GURL>& exceptions,
       const std::map<GURL, int64>& usage_map,
       int64 global_quota);
 
   content::BrowserContext* const browser_context_;
 
   DISALLOW_COPY_AND_ASSIGN(SiteEngagementEvictionPolicy);
 };
 
 #endif  // CHROME_BROWSER_ENGAGEMENT_SITE_ENGAGEMENT_EVICTION_POLICY_H_