[page_load_metrics] Add support for chrome:// urls and add extensions observer

chrome/browser/BUILD.gn

 # Copyright 2014 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.
 
 import("//build/config/chrome_build.gni")
 import("//build/config/crypto.gni")
 import("//build/config/features.gni")
 import("//build/config/ui.gni")
 import("//build/split_static_library.gni")
 import("//chrome/common/features.gni")
@@ skipping 692 matching lines @@
     "page_load_metrics/observers/https_engagement_metrics/https_engagement_service_factory.cc",
     "page_load_metrics/observers/https_engagement_metrics/https_engagement_service_factory.h",
     "page_load_metrics/observers/no_state_prefetch_page_load_metrics_observer.cc",
     "page_load_metrics/observers/no_state_prefetch_page_load_metrics_observer.h",
     "page_load_metrics/observers/previews_page_load_metrics_observer.cc",
     "page_load_metrics/observers/previews_page_load_metrics_observer.h",
     "page_load_metrics/observers/service_worker_page_load_metrics_observer.cc",
     "page_load_metrics/observers/service_worker_page_load_metrics_observer.h",
     "page_load_metrics/observers/stale_while_revalidate_metrics_observer.cc",
     "page_load_metrics/observers/stale_while_revalidate_metrics_observer.h",
+    "page_load_metrics/observers/webui_page_load_metrics_observer.cc",
+    "page_load_metrics/observers/webui_page_load_metrics_observer.h",
     "page_load_metrics/page_load_metrics_initialize.cc",
     "page_load_metrics/page_load_metrics_initialize.h",
     "page_load_metrics/page_load_metrics_observer.cc",
     "page_load_metrics/page_load_metrics_observer.h",
     "page_load_metrics/page_load_metrics_util.cc",
     "page_load_metrics/page_load_metrics_util.h",
     "password_manager/chrome_password_manager_client.cc",
     "password_manager/chrome_password_manager_client.h",
     "password_manager/password_manager_setting_migrator_service_factory.cc",
     "password_manager/password_manager_setting_migrator_service_factory.h",
@@ skipping 3516 matching lines @@
   # linking all of the test support.
   static_library("pepper_cdm_test_constants") {
     testonly = true
     visibility = [ "//chrome/*" ]
     sources = [
       "media/pepper_cdm_test_constants.cc",
       "media/pepper_cdm_test_constants.h",
     ]
   }
 }