Initialize OOBE/login WebUI in hidden state in parallel with wallpaper animation.

chrome/common/chrome_switches.cc

Index: chrome/common/chrome_switches.cc
diff --git a/chrome/common/chrome_switches.cc b/chrome/common/chrome_switches.cc
index 4e24a2db01096927f02eca7396d0fa945ed8c139..cb126f1dd7344b01aa26023ac41540ebabec213f 100644
--- a/chrome/common/chrome_switches.cc
+++ b/chrome/common/chrome_switches.cc
@@ -1349,6 +1349,11 @@ const char kTabletUI[]                      = "tablet-ui";
 #endif
 
 #if defined(OS_CHROMEOS)
+// When wallpaper boot animation is not disabled this switch
+// is used to override OOBE/sign in WebUI init type.
+// Possible values: parallel|postpone. Default: parallel.
+const char kAshWebUIInit[]                  = "ash-webui-init";
+
 // Disables wallpaper boot animation (except of OOBE case).
 const char kDisableBootAnimation[]          = "disable-boot-animation";