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

chrome/browser/about_flags.cc

Index: chrome/browser/about_flags.cc
diff --git a/chrome/browser/about_flags.cc b/chrome/browser/about_flags.cc
index 0a30e2daef3db931970d48eeb479509ccc244be5..b1af394fed82fcea1d896f7f86d5e3885a33ce4e 100644
--- a/chrome/browser/about_flags.cc
+++ b/chrome/browser/about_flags.cc
@@ -824,6 +824,13 @@ const Experiment kExperiments[] = {
     kOsCrOS,
     SINGLE_VALUE_TYPE(switches::kDisableNewOobe),
   },
+  {
+    "disable-boot-animation",
+    IDS_FLAGS_DISABLE_BOOT_ANIMATION,
+    IDS_FLAGS_DISABLE_BOOT_ANIMATION_DESCRIPTION,
+    kOsCrOS,
+    SINGLE_VALUE_TYPE(switches::kDisableBootAnimation),
+  },
 #endif
   {
     "enable-views-textfield",