Make YouTube default web app open in a window by default for new Chrome OS users

This CL updates the YouTube default web app to install as opening in a
window.
This only affects new users on Chrome OS.

Before:
https://bugs.chromium.org/p/chromium/issues/attachment?aid=481158&signed_aid=oxtHhds0ncHcPrMr76xdvA==&inline=1

After:
https://bugs.chromium.org/p/chromium/issues/attachment?aid=481159&signed_aid=B2AAZpgZRJqHpeXt61yyrw==&inline=1

Bug: 1159306
Change-Id: I59b3f83c801596520c37995d849a156e4c0fd89c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2592076
Commit-Queue: Alan Cutter <alancutter@chromium.org>
Reviewed-by: Glen Robertson <glenrob@chromium.org>
Cr-Commit-Position: refs/heads/master@{#837488}

chrome/browser/web_applications/preinstalled_web_apps/youtube.cc

@@ -17,7 +17,11 @@ ExternalInstallOptions GetConfigForYouTube() {
   ExternalInstallOptions options(
       /*install_url=*/GURL(
           "https://www.youtube.com/s/notifications/manifest/cr_install.html"),
+#if BUILDFLAG(IS_CHROMEOS_ASH) || BUILDFLAG(IS_CHROMEOS_LACROS)
+      /*user_display_mode=*/DisplayMode::kStandalone,
+#else
       /*user_display_mode=*/DisplayMode::kBrowser,
+#endif  // BUILDFLAG(IS_CHROMEOS_ASH) || BUILDFLAG(IS_CHROMEOS_LACROS)
       /*install_source=*/ExternalInstallSource::kExternalDefault);
 
   options.user_type_allowlist = {"unmanaged", "managed"};