Convert spellcheck to a buildflag header.

chrome/browser/about_flags.cc

Index: chrome/browser/about_flags.cc
diff --git a/chrome/browser/about_flags.cc b/chrome/browser/about_flags.cc
index 271f618a372c582235f010fc487ce5795a1b47ed..12c0c76df9f7bd5aab712701532b975f21fcabfa 100644
--- a/chrome/browser/about_flags.cc
+++ b/chrome/browser/about_flags.cc
@@ -56,6 +56,7 @@
 #include "components/signin/core/common/signin_switches.h"
 #include "components/spellcheck/common/spellcheck_features.h"
 #include "components/spellcheck/common/spellcheck_switches.h"
+#include "components/spellcheck/spellcheck_build_features.h"
 #include "components/ssl_config/ssl_config_switches.h"
 #include "components/strings/grit/components_strings.h"
 #include "components/sync/driver/sync_driver_switches.h"
@@ -844,7 +845,7 @@ const FeatureEntry kFeatureEntries[] = {
      IDS_FLAGS_SILENT_DEBUGGER_EXTENSION_API_NAME,
      IDS_FLAGS_SILENT_DEBUGGER_EXTENSION_API_DESCRIPTION, kOsDesktop,
      SINGLE_VALUE_TYPE(switches::kSilentDebuggerExtensionAPI)},
-#if defined(ENABLE_SPELLCHECK) && defined(OS_ANDROID)
+#if BUILDFLAG(ENABLE_SPELLCHECK) && defined(OS_ANDROID)
     {"enable-android-spellchecker", IDS_OPTIONS_ENABLE_SPELLCHECK,
      IDS_OPTIONS_ENABLE_ANDROID_SPELLCHECKER_DESCRIPTION, kOsAndroid,
      FEATURE_VALUE_TYPE(spellcheck::kAndroidSpellChecker)},
@@ -1198,7 +1199,7 @@ const FeatureEntry kFeatureEntries[] = {
      SINGLE_DISABLE_VALUE_TYPE(
          switches::kDisableHideInactiveStackedTabCloseButtons)},
 #endif  // TOOLKIT_VIEWS
-#if defined(ENABLE_SPELLCHECK)
+#if BUILDFLAG(ENABLE_SPELLCHECK)
     {"enable-spelling-feedback-field-trial",
      IDS_FLAGS_SPELLING_FEEDBACK_FIELD_TRIAL_NAME,
      IDS_FLAGS_SPELLING_FEEDBACK_FIELD_TRIAL_DESCRIPTION, kOsAll,