Convert //ios/chrome/browser:about_flags to use buildflag_header.

ios/chrome/browser/ui/webui/BUILD.gn

 # Copyright 2016 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.
 
 source_set("webui") {
   configs += [ "//build/config/compiler:enable_arc" ]
   sources = [
     "about_ui.cc",
     "about_ui.h",
     "crashes_ui.cc",
@@ skipping 20 matching lines @@
     "//components/physical_web/data_source",
     "//components/physical_web/webui",
     "//components/prefs",
     "//components/resources",
     "//components/strings",
     "//components/version_info",
     "//components/version_ui",
     "//google_apis",
     "//ios/chrome/app/strings",
     "//ios/chrome/browser",
-    "//ios/chrome/browser:about_flags",
     "//ios/chrome/browser/browser_state",
     "//ios/chrome/browser/crash_report",
     "//ios/chrome/browser/metrics",
     "//ios/chrome/browser/ntp_tiles",
     "//ios/chrome/browser/ui",
     "//ios/chrome/common",
     "//ios/web",
     "//net",
     "//services/service_manager/public/cpp",
     "//third_party/brotli",
     "//ui/base",
     "//url",
   ]
 }