Convert spellcheck to a buildflag header.

components/spellcheck/spellcheck_build_features.gni

Index: components/spellcheck/spellcheck_build_features.gni
diff --git a/components/spellcheck/spellcheck_build_features.gni b/components/spellcheck/spellcheck_build_features.gni
new file mode 100644
index 0000000000000000000000000000000000000000..ef26aea4a7141fa516d14acb6130f54ceb706efc
--- /dev/null
+++ b/components/spellcheck/spellcheck_build_features.gni
@@ -0,0 +1,11 @@
+# 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.
+
+# Enable the spell checker.
+enable_spellcheck = !is_ios
+
+# Use the operating system's spellchecker rather than hunspell. This does
+# not affect the "red underline" spellchecker which can consult Google's
+# server-based spellcheck service.
+use_browser_spellchecker = is_android || is_mac