Enable wallpaper boot animation for boot into sign in too. Add flag to disable it.

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 32049472f71dbc9250832d4f75e54a4748e0acce..b8f47f0fce40dca65788ffd4a745467376883042 100644
--- a/chrome/common/chrome_switches.cc
+++ b/chrome/common/chrome_switches.cc
@@ -1340,6 +1340,9 @@ const char kTabletUI[]                      = "tablet-ui";
 #endif
 
 #if defined(OS_CHROMEOS)
+// Disables wallpaper boot animation (except of OOBE case).
+const char kDisableBootAnimation[]          = "disable-boot-animation";
+
 // Disables gdata content provider.
 const char kDisableGData[]                  = "disable-gdata";