Plumb experiment flags through page_load_metrics and add a new observer

chrome/chrome_tests_unit.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': {
     'chrome_unit_tests_sources': [
       # histograms.xml is analyzed by AboutFlagsHistogramTest, so this
       # dependency is needed to make commit bots run unit_tests on
       # histograms.xml changes.
       '../tools/metrics/histograms/histograms.xml',
@@ skipping 135 matching lines @@
       'browser/net/net_error_tab_helper_unittest.cc',
       'browser/net/predictor_unittest.cc',
       'browser/net/probe_message_unittest.cc',
       'browser/net/quota_policy_channel_id_store_unittest.cc',
       'browser/net/safe_search_util_unittest.cc',
       'browser/net/spdyproxy/data_reduction_proxy_chrome_settings_unittest.cc',
       'browser/net/spdyproxy/data_reduction_proxy_settings_unittest_android.cc',
       'browser/net/url_info_unittest.cc',
       'browser/page_load_metrics/observers/aborts_page_load_metrics_observer_unittest.cc',
       'browser/page_load_metrics/observers/core_page_load_metrics_observer_unittest.cc',
+      'browser/page_load_metrics/observers/document_write_page_load_metrics_observer_unittest.cc',
       'browser/page_load_metrics/observers/from_gws_page_load_metrics_observer_unittest.cc',
       'browser/page_load_metrics/observers/google_captcha_observer_unittest.cc',
       'browser/page_load_metrics/observers/page_load_metrics_observer_test_harness.cc',
       'browser/page_load_metrics/observers/page_load_metrics_observer_test_harness.h',
       'browser/password_manager/chrome_password_manager_client_unittest.cc',
       'browser/password_manager/password_manager_internals_service_unittest.cc',
       'browser/password_manager/password_store_mac_unittest.cc',
       'browser/password_manager/password_store_proxy_mac_unittest.cc',
       'browser/password_manager/password_store_win_unittest.cc',
       'browser/password_manager/password_store_x_unittest.cc',
@@ skipping 2834 matching lines @@
               'dependencies': [
                 '../tools/xdisplaycheck/xdisplaycheck.gyp:xdisplaycheck',
               ],
             }],
           ],
         },
       ],
     }],
   ],  # 'conditions'
 }