Convert spellcheck to a buildflag header.

android_webview/native/android_webview_jni_registrar.cc

Index: android_webview/native/android_webview_jni_registrar.cc
diff --git a/android_webview/native/android_webview_jni_registrar.cc b/android_webview/native/android_webview_jni_registrar.cc
index f3b20ea4e15166f62d735331a153ab64a39ee118..7697aa142feedd7e49385d4fe12ddf4571f86b41 100644
--- a/android_webview/native/android_webview_jni_registrar.cc
+++ b/android_webview/native/android_webview_jni_registrar.cc
@@ -25,8 +25,9 @@
 #include "base/android/jni_android.h"
 #include "base/android/jni_registrar.h"
 #include "base/trace_event/trace_event.h"
+#include "components/spellcheck/spellcheck_build_features.h"
 
-#if defined(ENABLE_SPELLCHECK)
+#if BUILDFLAG(ENABLE_SPELLCHECK)
 #include "components/spellcheck/browser/android/component_jni_registrar.h"
 #endif
 
@@ -52,7 +53,7 @@ static base::android::RegistrationMethod kWebViewRegisteredMethods[] = {
   { "AwWebContentsDelegate", RegisterAwWebContentsDelegate },
   { "CookieManager", RegisterCookieManager },
   { "AwGLFunctor", RegisterAwGLFunctor },
-#if defined(ENABLE_SPELLCHECK)
+#if BUILDFLAG(ENABLE_SPELLCHECK)
   {"SpellCheckerSessionBridge", spellcheck::android::RegisterSpellcheckJni},
 #endif
 };