Factor PageLoadTracker into its own header and impl files.

chrome/browser/BUILD.gn

Index: chrome/browser/BUILD.gn
diff --git a/chrome/browser/BUILD.gn b/chrome/browser/BUILD.gn
index 9a51a0f8330f4c658639c9ffab4bb7fbc2a2b71e..9171a9c9faec83065d292ca9dc47a4f0640e406f 100644
--- a/chrome/browser/BUILD.gn
+++ b/chrome/browser/BUILD.gn
@@ -717,12 +717,15 @@ split_static_library("browser") {
     "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/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",
+    "page_load_metrics/page_load_tracker.h",
     "password_manager/chrome_password_manager_client.cc",
     "password_manager/chrome_password_manager_client.h",
     "password_manager/password_manager_setting_migrator_service_factory.cc",