Implement an observer interface for the site engagement service.

chrome/chrome_browser.gypi

 # Copyright 2012 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.
 
 {
   'variables': {
     # All .cc, .h, .m, and .mm files under browser except for:
     # * tests and mocks.
     # * code below browser/chromeos
     # * code below browser/extensions
@@ skipping 1622 matching lines @@
       'browser/data_usage/tab_id_provider.cc',
       'browser/data_usage/tab_id_provider.h',
     ],
     'chrome_browser_engagement_sources': [
       'browser/engagement/site_engagement_eviction_policy.cc',
       'browser/engagement/site_engagement_eviction_policy.h',
       'browser/engagement/site_engagement_helper.cc',
       'browser/engagement/site_engagement_helper.h',
       'browser/engagement/site_engagement_metrics.cc',
       'browser/engagement/site_engagement_metrics.h',
+      'browser/engagement/site_engagement_observer.cc',
+      'browser/engagement/site_engagement_observer.h',
       'browser/engagement/site_engagement_score.cc',
       'browser/engagement/site_engagement_score.h',
       'browser/engagement/site_engagement_service.cc',
       'browser/engagement/site_engagement_service.h',
       'browser/engagement/site_engagement_service_factory.cc',
       'browser/engagement/site_engagement_service_factory.h',
     ],
     'chrome_browser_extensions_sources': [
       'browser/accessibility/accessibility_extension_api.cc',
       'browser/accessibility/accessibility_extension_api.h',
@@ skipping 2606 matching lines @@
             '../components/components.gyp:rlz',
             '../components/components.gyp:search_engines',
             '../rlz/rlz.gyp:rlz_lib',
             'common',
           ],
         },
       ],
     }],
   ],
 }