Add an external_startup_data config to the GN buildfile.

BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/config/android/config.gni")
 import("//build/config/arm.gni")
 import("//build/config/mips.gni")
 import("//build/config/sanitizers/sanitizers.gni")
 
 # Because standalone V8 builds are not supported, assume this is part of a
@@ skipping 53 matching lines @@
 config("external_config") {
   if (is_component_build) {
     defines = [
       "V8_SHARED",
       "USING_V8_SHARED",
     ]
   }
   include_dirs = [ "include" ]
 }
 
+# This config should only be applied to code that needs to be explicitly
+# aware of whether we are using startup data or not.
+config("external_startup_data") {
+  if (v8_use_external_startup_data) {
+    defines = [ "V8_USE_EXTERNAL_STARTUP_DATA" ]
+  }
+}
+
 config("features") {
   visibility = [ ":*" ]  # Only targets in this file can depend on this.
 
   defines = []
 
   if (v8_enable_disassembler == true) {
     defines += [ "ENABLE_DISASSEMBLER" ]
   }
   if (v8_enable_gdbjit == true) {
     defines += [ "ENABLE_GDB_JIT_INTERFACE" ]
@@ skipping 1742 matching lines @@
     if (!is_component_build) {
       sources += [
         "$target_gen_dir/d8-js.cc",
       ]
     }
     if (v8_enable_i18n_support) {
       deps += [ "//third_party/icu" ]
     }
   }
 }