Added UkmPageLoadMetricsObserver which sends top-level metrics to UKM

chrome/browser/BUILD.gn

Index: chrome/browser/BUILD.gn
diff --git a/chrome/browser/BUILD.gn b/chrome/browser/BUILD.gn
index 443d291c4d75874972f58ff4f1d5d99d1edef86d..118bea9367e96bd8c13d6e3dde15ea0fefbbac8e 100644
--- a/chrome/browser/BUILD.gn
+++ b/chrome/browser/BUILD.gn
@@ -802,6 +802,8 @@ split_static_library("browser") {
     "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/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",
@@ -4292,6 +4294,7 @@ static_library("test_support") {
     "//components/subresource_filter/core/browser:test_support",
     "//components/subresource_filter/core/common:test_support",
     "//components/sync_preferences:test_support",
+    "//components/ukm:test_support",
     "//components/user_prefs/tracked:user_prefs_tracked_test_support",
     "//content/test:test_support",
     "//google_apis:test_support",