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

chrome/browser/resources/chromeos/login/user_pod_row.js

Index: chrome/browser/resources/chromeos/login/user_pod_row.js
diff --git a/chrome/browser/resources/chromeos/login/user_pod_row.js b/chrome/browser/resources/chromeos/login/user_pod_row.js
index 83a497883a1247727aaf6b6bedd79d16ebac2645..3ab40dd2fc593101baf4edc1b0d8a05a94f4f5d5 100644
--- a/chrome/browser/resources/chromeos/login/user_pod_row.js
+++ b/chrome/browser/resources/chromeos/login/user_pod_row.js
@@ -33,7 +33,7 @@ cr.define('login', function() {
    * @type {number}
    * @const
    */
-  var WALLPAPER_BOAT_LOAD_DELAY_MS = 500;
+  var WALLPAPER_BOOT_LOAD_DELAY_MS = 500;
 
   /**
    * Oauth token status. These must match UserManager::OAuthTokenStatus.
@@ -679,8 +679,10 @@ cr.define('login', function() {
             // Boot transition. Delay wallpaper load to remove jank
             // happening when wallpaper load is competing for resources with
             // login WebUI.
-            this.loadWallpaperTimeout_ = window.setTimeout(
-                this.loadWallpaper_.bind(this), WALLPAPER_BOAT_LOAD_DELAY_MS);
+            if (Oobe.getInstance().shouldLoadWallpaperOnBoot()) {
+              this.loadWallpaperTimeout_ = window.setTimeout(
+                  this.loadWallpaper_.bind(this), WALLPAPER_BOOT_LOAD_DELAY_MS);
+            }
             this.firstShown_ = false;
           }
         }