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

chrome/browser/chrome_browser_main_extra_parts_exo.cc

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/chrome_browser_main_extra_parts_exo.h"
 
 #include "base/memory/ptr_util.h"
 
 #if defined(USE_GLIB)
 #include <glib.h>
@@ skipping 113 matching lines @@
   arc_notification_surface_manager_ =
       base::MakeUnique<arc::ArcNotificationSurfaceManager>();
   if (chrome::IsRunningInMash())
     wm_helper_ = base::MakeUnique<exo::WMHelperMus>();
   else
     wm_helper_ = base::MakeUnique<exo::WMHelperAsh>();
   exo::WMHelper::SetInstance(wm_helper_.get());
   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());
 }
 
 void ChromeBrowserMainExtraPartsExo::PostMainMessageLoopRun() {
   wayland_watcher_.reset();
   wayland_server_.reset();
   if (wm_helper_) {
     exo::WMHelper::SetInstance(nullptr);
     wm_helper_.reset();
   }
 }