Add PageLoad metrics for multi-tab loading cases

chrome/test/BUILD.gn

Index: chrome/test/BUILD.gn
diff --git a/chrome/test/BUILD.gn b/chrome/test/BUILD.gn
index 0515c2c0ebf8f0db47292570229c77730889f258..de7c8372b6a12ce950e4c9f2d43b7570a3787d26 100644
--- a/chrome/test/BUILD.gn
+++ b/chrome/test/BUILD.gn
@@ -1349,6 +1349,7 @@ test("browser_tests") {
       "../browser/ntp_snippets/content_suggestions_service_factory_browsertest.cc",
       "../browser/page_load_metrics/observers/ads_page_load_metrics_observer_browsertest.cc",
       "../browser/page_load_metrics/observers/https_engagement_metrics/https_engagement_page_load_metrics_observer_browsertest.cc",
+      "../browser/page_load_metrics/observers/multi_tab_loading_page_load_metrics_observer_browsertest.cc",
       "../browser/page_load_metrics/page_load_metrics_browsertest.cc",
       "../browser/password_manager/credential_manager_browsertest.cc",
       "../browser/password_manager/password_manager_browsertest.cc",
@@ -3164,6 +3165,7 @@ test("unit_tests") {
     "../browser/page_load_metrics/observers/loading_predictor_page_load_metrics_observer_unittest.cc",
     "../browser/page_load_metrics/observers/lofi_page_load_metrics_observer_unittest.cc",
     "../browser/page_load_metrics/observers/media_page_load_metrics_observer_unittest.cc",
+    "../browser/page_load_metrics/observers/multi_tab_loading_page_load_metrics_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/page_load_metrics/observers/previews_page_load_metrics_observer_unittest.cc",