Remove OTIFICATION_METRIC_EVENT_DURATION that no-one is observing

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 661 matching lines @@
       'browser/metrics/chrome_metrics_service_accessor.cc',
       'browser/metrics/chrome_metrics_service_accessor.h',
       'browser/metrics/chrome_metrics_service_client.cc',
       'browser/metrics/chrome_metrics_service_client.h',
       'browser/metrics/chrome_stability_metrics_provider.cc',
       'browser/metrics/chrome_stability_metrics_provider.h',
       'browser/metrics/field_trial_synchronizer.cc',
       'browser/metrics/field_trial_synchronizer.h',
       'browser/metrics/google_update_metrics_provider_win.cc',
       'browser/metrics/google_update_metrics_provider_win.h',
-      'browser/metrics/metric_event_duration_details.h',
       'browser/metrics/metrics_reporting_state.cc',
       'browser/metrics/metrics_reporting_state.h',
       'browser/metrics/metrics_service_accessor.cc',
       'browser/metrics/metrics_service_accessor.h',
       'browser/metrics/metrics_services_manager.cc',
       'browser/metrics/metrics_services_manager.h',
       'browser/metrics/network_stats_uploader.cc',
       'browser/metrics/network_stats_uploader.h',
       'browser/metrics/omnibox_metrics_provider.cc',
       'browser/metrics/omnibox_metrics_provider.h',
@@ skipping 2911 matching lines @@
           'variables': {
             'proto_in_dir': 'browser/sync_file_system/drive_backend',
             'proto_out_dir': 'chrome/browser/sync_file_system/drive_backend',
           },
           'includes': [ '../build/protoc.gypi' ]
         },
       ],
     },],
   ],
 }