Remove non-cros support from ash/common/system.

ash/common/system/web_notification/web_notification_tray.cc

Index: ash/common/system/web_notification/web_notification_tray.cc
diff --git a/ash/common/system/web_notification/web_notification_tray.cc b/ash/common/system/web_notification/web_notification_tray.cc
index b6814039a77f344028998afda57d9a4733093c54..140276f98f41fd32cc39606d1e385b67eb30d7fc 100644
--- a/ash/common/system/web_notification/web_notification_tray.cc
+++ b/ash/common/system/web_notification/web_notification_tray.cc
@@ -45,20 +45,16 @@
 #include "ui/views/controls/menu/menu_runner.h"
 #include "ui/views/layout/fill_layout.h"
 
-#if defined(OS_CHROMEOS)
-
 namespace message_center {
 
 MessageCenterTrayDelegate* CreateMessageCenterTray() {
-  // On Windows+Ash the Tray will not be hosted in ash::Shell.
+  // On non-CrOS, the Tray will not be hosted in ash::Shell.
   NOTREACHED();
   return nullptr;
 }
 
 }  // namespace message_center
 
-#endif  // defined(OS_CHROMEOS)
-
 namespace ash {
 namespace {