Convert spellcheck to a buildflag header.

android_webview/BUILD.gn

Index: android_webview/BUILD.gn
diff --git a/android_webview/BUILD.gn b/android_webview/BUILD.gn
index f5d8c7354c809d2f53e6fdd34b08a0ece29b37ef..a0e40238e6a2677465bc130f61fcc0f1cc42ab10 100644
--- a/android_webview/BUILD.gn
+++ b/android_webview/BUILD.gn
@@ -2,13 +2,14 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import("//android_webview/system_webview_apk_tmpl.gni")
+import("//android_webview/webview_repack_locales.gni")
 import("//build/config/android/config.gni")
 import("//build/config/android/rules.gni")
 import("//build/config/locales.gni")
+import("//components/spellcheck/spellcheck_build_features.gni")
 import("//tools/grit/repack.gni")
 import("//tools/resources/generate_resource_whitelist.gni")
-import("system_webview_apk_tmpl.gni")
-import("webview_repack_locales.gni")
 
 declare_args() {
   # Package name of the system_webview_apk target.
@@ -488,6 +489,7 @@ source_set("common") {
     "//components/printing/browser",
     "//components/printing/common",
     "//components/printing/renderer",
+    "//components/spellcheck:build_features",
     "//components/supervised_user_error_page",
     "//components/supervised_user_error_page:gin",
     "//components/version_info",