Convert spellcheck to a buildflag header.

components/spellcheck/browser/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.
 
 if (is_android) {
   import("//build/config/android/rules.gni")
 }
 
 source_set("browser") {
   sources = [
@@ skipping 18 matching lines @@
     "spellchecker_session_bridge_android.cc",
     "spellchecker_session_bridge_android.h",
     "spelling_service_client.cc",
     "spelling_service_client.h",
     "word_trimmer.cc",
     "word_trimmer.h",
   ]
 
   public_deps = [
     "//base",
+    "//components/spellcheck:build_features",
     "//components/spellcheck/common",
   ]
   deps = [
     "//components/data_use_measurement/core",
     "//components/prefs",
     "//components/user_prefs",
     "//content/public/browser",
     "//content/public/common",
     "//google_apis",
     "//net",
@@ skipping 24 matching lines @@
     ":browser",
     "//base",
     "//base/test:test_support",
     "//components/spellcheck/common",
     "//components/variations",
     "//content/test:test_support",
     "//net:test_support",
     "//testing/gtest",
   ]
 }