exo: Add comment explaining why wayland server creation can fail.

chrome/browser/chrome_browser_main_extra_parts_exo.cc

Index: chrome/browser/chrome_browser_main_extra_parts_exo.cc
diff --git a/chrome/browser/chrome_browser_main_extra_parts_exo.cc b/chrome/browser/chrome_browser_main_extra_parts_exo.cc
index 01df0be806d056ddbd2a5b175421589fd6df5c5c..af9342b4f2fb93cba1fcde390d7be4eaa7fea04a 100644
--- a/chrome/browser/chrome_browser_main_extra_parts_exo.cc
+++ b/chrome/browser/chrome_browser_main_extra_parts_exo.cc
@@ -131,6 +131,7 @@ void ChromeBrowserMainExtraPartsExo::PreProfileInit() {
   display_ =
       base::MakeUnique<exo::Display>(arc_notification_surface_manager_.get());
   wayland_server_ = exo::wayland::Server::Create(display_.get());
+  // Wayland server creation can fail if XDG_RUNTIME_DIR is not set correctly.
   if (wayland_server_)
     wayland_watcher_ = base::MakeUnique<WaylandWatcher>(wayland_server_.get());
 }