Add subresource filter page load metrics 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 783 matching lines @@
     "page_load_metrics/observers/prerender_page_load_metrics_observer.cc",
     "page_load_metrics/observers/prerender_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/protocol_page_load_metrics_observer.cc",
     "page_load_metrics/observers/protocol_page_load_metrics_observer.h",
     "page_load_metrics/observers/resource_prefetch_predictor_page_load_metrics_observer.cc",
     "page_load_metrics/observers/resource_prefetch_predictor_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/subresource_filter_metrics_observer.cc",
+    "page_load_metrics/observers/subresource_filter_metrics_observer.h",
     "page_load_metrics/observers/ukm_page_load_metrics_observer.cc",
     "page_load_metrics/observers/ukm_page_load_metrics_observer.h",
     "page_load_metrics/page_load_metrics_embedder_interface.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",
     "page_load_metrics/page_load_tracker.cc",
@@ skipping 3703 matching lines @@
       "media/pepper_cdm_test_constants.cc",
       "media/pepper_cdm_test_constants.h",
     ]
   }
 }
 
 service_manifest("preferences_manifest") {
   name = "preferences"
   source = "prefs/preferences_manifest.json"
 }