Move v8_use_external_startup_data out of //build/config:feature_flags.

build/config/BUILD.gn

Index: build/config/BUILD.gn
diff --git a/build/config/BUILD.gn b/build/config/BUILD.gn
index 64077ccf26eb8c96c87f03eef933b5afa9b966bb..6573cb472e7f38ce99c286bf8fe02adf4bb8cf7a 100644
--- a/build/config/BUILD.gn
+++ b/build/config/BUILD.gn
@@ -8,7 +8,6 @@ import("//build/config/crypto.gni")
 import("//build/config/features.gni")
 import("//build/config/sanitizers/sanitizers.gni")
 import("//build/config/ui.gni")
-import("//build_overrides/v8.gni")
 import("//build/toolchain/goma.gni")
 
 declare_args() {
@@ -243,9 +242,6 @@ config("feature_flags") {
   if (enable_hangout_services_extension) {
     defines += [ "ENABLE_HANGOUT_SERVICES_EXTENSION=1" ]
   }
-  if (v8_use_external_startup_data) {
-    defines += [ "V8_USE_EXTERNAL_STARTUP_DATA" ]
-  }
   if (enable_background) {
     defines += [ "ENABLE_BACKGROUND=1" ]
   }