Revert "build: remove buildflags workaround (#27372)" (#27410)

This reverts commit e4fc47f55701e53de6a16c31900aa48594315d73.

.circleci/config.yml

@@ -447,6 +447,10 @@ step-fix-sync-on-mac: &step-fix-sync-on-mac
         # Fix Clang Install (wrong binary)
         rm -rf src/third_party/llvm-build
         python src/tools/clang/scripts/update.py
+        # Temporarily use an older version of gn
+        unset CI
+        python src/buildtools/ensure_gn_version.py git_revision:53d92014bf94c3893886470a1c7c1289f8818db0
+        export CI=true
       fi
 
 step-install-signing-cert-on-mac: &step-install-signing-cert-on-mac

BUILD.gn

@@ -3,7 +3,6 @@ import("//build/config/ui.gni")
 import("//build/config/win/manifest.gni")
 import("//components/spellcheck/spellcheck_build_features.gni")
 import("//content/public/app/mac_helpers.gni")
-import("//electron/buildflags/buildflags.gni")
 import("//extensions/buildflags/buildflags.gni")
 import("//pdf/features.gni")
 import("//ppapi/buildflags/buildflags.gni")
@@ -21,6 +20,7 @@ import("build/npm.gni")
 import("build/templated_file.gni")
 import("build/tsc.gni")
 import("build/webpack/webpack.gni")
+import("buildflags/buildflags.gni")
 import("electron_paks.gni")
 import("filenames.auto.gni")
 import("filenames.gni")