Convert spellcheck to a buildflag header.

components/spellcheck/common/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("common") {
   sources = [
     "spellcheck_bdict_language.h",
     "spellcheck_common.cc",
     "spellcheck_common.h",
     "spellcheck_features.cc",
     "spellcheck_features.h",
     "spellcheck_marker.h",
     "spellcheck_message_generator.cc",
     "spellcheck_message_generator.h",
     "spellcheck_messages.h",
     "spellcheck_result.h",
     "spellcheck_switches.cc",
     "spellcheck_switches.h",
   ]
 
+  public_deps = [
+    "//components/spellcheck:build_features",
+  ]
   deps = [
     "//base:i18n",
     "//ipc",
     "//third_party/icu",
   ]
 }