[gn] Initialize gn for stand-alone v8

build_overrides/v8.gni

Index: build_overrides/v8.gni
diff --git a/build_overrides/v8.gni b/build_overrides/v8.gni
new file mode 100644
index 0000000000000000000000000000000000000000..d9d640c5fb057afa393e962478c02aeebeb83161
--- /dev/null
+++ b/build_overrides/v8.gni
@@ -0,0 +1,15 @@
+# Copyright 2015 The V8 project 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/features.gni")
+import("//build/config/ui.gni")
+
+if (is_android) {
+  import("//build/config/android/config.gni")
+}
+
+v8_use_external_startup_data = false
+v8_optimized_debug = false
+v8_extra_library_files = []
+v8_experimental_extra_library_files = []