Convert spellcheck to a buildflag header.

components/spellcheck/renderer/hunspell_engine.cc

Index: components/spellcheck/renderer/hunspell_engine.cc
diff --git a/components/spellcheck/renderer/hunspell_engine.cc b/components/spellcheck/renderer/hunspell_engine.cc
index 191856541d819234c1b48fc227ac6be4fd68d74b..8617aaa2185135f9e7b86f73e9535c600d834734 100644
--- a/components/spellcheck/renderer/hunspell_engine.cc
+++ b/components/spellcheck/renderer/hunspell_engine.cc
@@ -13,6 +13,7 @@
 #include "base/time/time.h"
 #include "components/spellcheck/common/spellcheck_common.h"
 #include "components/spellcheck/common/spellcheck_messages.h"
+#include "components/spellcheck/spellcheck_build_features.h"
 #include "content/public/renderer/render_thread.h"
 #include "third_party/hunspell/src/hunspell/hunspell.hxx"
 
@@ -33,7 +34,7 @@ namespace {
                 "MaxSuggestLen too long");
 }  // namespace
 
-#if !defined(USE_BROWSER_SPELLCHECKER)
+#if !BUILDFLAG(USE_BROWSER_SPELLCHECKER)
 SpellingEngine* CreateNativeSpellingEngine() {
   return new HunspellEngine();
 }