exo: Avoid crash at startup when XDG_RUNTIME_DIR is not set.

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 8221f549090b4ddb09674233ace0da25791c5b05..01df0be806d056ddbd2a5b175421589fd6df5c5c 100644
--- a/chrome/browser/chrome_browser_main_extra_parts_exo.cc
+++ b/chrome/browser/chrome_browser_main_extra_parts_exo.cc
@@ -131,7 +131,8 @@ void ChromeBrowserMainExtraPartsExo::PreProfileInit() {
   display_ =
       base::MakeUnique<exo::Display>(arc_notification_surface_manager_.get());
   wayland_server_ = exo::wayland::Server::Create(display_.get());
-  wayland_watcher_ = base::MakeUnique<WaylandWatcher>(wayland_server_.get());
+  if (wayland_server_)

[sky, 2016/12/12 02:18:38]

Maybe add a comment as to why the null check?

[reveman, 2016/12/12 11:37:36]

Done: https://codereview.chromium.org/2562363002

+    wayland_watcher_ = base::MakeUnique<WaylandWatcher>(wayland_server_.get());
 }
 
 void ChromeBrowserMainExtraPartsExo::PostMainMessageLoopRun() {