Adding client code for new desktop First Run Experience.

chrome/browser/ui/BUILD.gn

Index: chrome/browser/ui/BUILD.gn
diff --git a/chrome/browser/ui/BUILD.gn b/chrome/browser/ui/BUILD.gn
index 1279d6bff0ffbdf620a6d8ee8f12997ffef3632a..20dffa7c5edf37e3c10bd1cccd3f0fb822f4e5d1 100644
--- a/chrome/browser/ui/BUILD.gn
+++ b/chrome/browser/ui/BUILD.gn
@@ -1923,6 +1923,8 @@ split_static_library("ui") {
       "webui/signin/user_manager_screen_handler.h",
       "webui/signin/user_manager_ui.cc",
       "webui/signin/user_manager_ui.h",
+      "webui/welcome_ui.cc",
+      "webui/welcome_ui.h",
     ]
     if (is_mac && !mac_views_browser) {
       sources -= [