Convert spellcheck to a buildflag header.

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",
+  ]
+}