Make SpellCheckPanel compiled and used only on Mac

components/spellcheck/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.
 
 import("//build/buildflag_header.gni")
 import("//components/spellcheck/spellcheck_build_features.gni")
 
 buildflag_header("build_features") {
   # Name this "build" features to avoid confusion with
   # components/spellcheck/common/spellcheck_features.h which are runtime
   # features.
   header = "spellcheck_build_features.h"
   flags = [
     "ENABLE_SPELLCHECK=$enable_spellcheck",
     "USE_BROWSER_SPELLCHECKER=$use_browser_spellchecker",
+    "HAS_SPELLCHECK_PANEL=$has_spellcheck_panel",
   ]
 }