Plumb experiment flags through page_load_metrics and add a new observer

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 393 matching lines @@
       'browser/mod_pagespeed/mod_pagespeed_metrics.h',
       'browser/native_window_notification_source.h',
       'browser/net/predictor_tab_helper.cc',
       'browser/net/predictor_tab_helper.h',
       'browser/ntp_snippets/ntp_snippets_service_factory.cc',
       'browser/ntp_snippets/ntp_snippets_service_factory.h',
       'browser/page_load_metrics/observers/aborts_page_load_metrics_observer.cc',
       'browser/page_load_metrics/observers/aborts_page_load_metrics_observer.h',
       'browser/page_load_metrics/observers/core_page_load_metrics_observer.cc',
       'browser/page_load_metrics/observers/core_page_load_metrics_observer.h',
+      'browser/page_load_metrics/observers/document_write_page_load_metrics_observer.cc',
+      'browser/page_load_metrics/observers/document_write_page_load_metrics_observer.h',
       'browser/page_load_metrics/observers/from_gws_page_load_metrics_observer.cc',
       'browser/page_load_metrics/observers/from_gws_page_load_metrics_observer.h',
       'browser/page_load_metrics/observers/google_captcha_observer.cc',
       'browser/page_load_metrics/observers/google_captcha_observer.h',
       'browser/page_load_metrics/observers/stale_while_revalidate_metrics_observer.cc',
       'browser/page_load_metrics/observers/stale_while_revalidate_metrics_observer.h',
       'browser/page_load_metrics/page_load_metrics_initialize.cc',
       'browser/page_load_metrics/page_load_metrics_initialize.h',
       'browser/performance_monitor/performance_monitor.cc',
       'browser/performance_monitor/performance_monitor.h',
@@ skipping 3783 matching lines @@
             '../components/components.gyp:rlz',
             '../components/components.gyp:search_engines',
             '../rlz/rlz.gyp:rlz_lib',
             'common',
           ],
         },
       ],
     }],
   ],
 }