[gn] Set correct defaults for some gn args

build_overrides/v8.gni

 # 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")
 import("//build/config/v8_target_cpu.gni")
 import("//gni/v8.gni")
 
 if (is_android) {
   import("//build/config/android/config.gni")
 }
 
 if (((v8_target_cpu == "x86" ||
       v8_target_cpu == "x64" ||
       v8_target_cpu=="x87") &&
      (is_linux || is_mac)) ||
     (v8_target_cpu == "ppc64" && is_linux)) {
-  v8_enable_gdbjit = true
+  v8_enable_gdbjit_default = true
 }
 
-v8_imminent_deprecation_warnings = true
+v8_imminent_deprecation_warnings_default = true
 
 # Add simple extras solely for the purpose of the cctests.
 v8_extra_library_files = [ "//test/cctest/test-extra.js" ]
 v8_experimental_extra_library_files = [ "//test/cctest/test-experimental-extra.js" ]