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

chrome/browser/about_flags.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/about_flags.h"
 
 #include <algorithm>
 #include <iterator>
 #include <map>
 #include <set>
@@ skipping 806 matching lines @@
     kOsCrOS,
     SINGLE_VALUE_TYPE(switches::kDisableHtml5Camera),
   },
   {
     "disable-new-oobe",
     IDS_FLAGS_DISABLE_NEW_OOBE,
     IDS_FLAGS_DISABLE_NEW_OOBE_DESCRIPTION,
     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",
     IDS_FLAGS_ENABLE_VIEWS_TEXTFIELD_NAME,
     IDS_FLAGS_ENABLE_VIEWS_TEXTFIELD_DESCRIPTION,
     kOsWin,
     SINGLE_VALUE_TYPE(switches::kEnableViewsTextfield),
   },
   {
     "old-checkbox-style",
@@ skipping 469 matching lines @@
 }
 
 const Experiment* GetExperiments(size_t* count) {
   *count = num_experiments;
   return experiments;
 }
 
 }  // namespace testing
 
 }  // namespace about_flags