ChromeOS user_manager: move all KnownUser code to separate file.

components/user_manager/BUILD.gn

 # Copyright 2014 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.
 
 component("user_manager") {
   sources = [
     "empty_user_info.cc",
     "empty_user_info.h",
     "user_info.cc",
     "user_info.h",
     "user_info_impl.cc",
     "user_info_impl.h",
     "user_manager_export.h",
   ]
 
   deps = [
     "//base",
     "//components/signin/core/account_id",
     "//skia",
     "//ui/gfx",
   ]
 
   defines = [ "USER_MANAGER_IMPLEMENTATION" ]
 
   if (is_chromeos) {
     sources += [
+      "known_user.cc",
+      "known_user.h",
       "remove_user_delegate.h",
       "user.cc",
       "user.h",
       "user_image/default_user_images.cc",
       "user_image/default_user_images.h",
       "user_image/user_image.cc",
       "user_image/user_image.h",
       "user_manager.cc",
       "user_manager.h",
       "user_manager_base.cc",
@@ skipping 22 matching lines @@
     ]
     deps = [
       ":user_manager",
       "//base",
       "//components/signin/core/account_id",
       "//skia",
       "//ui/base",
     ]
   }
 }