[headless] Use embedded v8 data for headless.

build_overrides/v8.gni

 # Copyright 2015 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.
 
+declare_args() {
+  v8_use_external_startup_data = !is_ios
+}
+
 if (is_android) {
   import("//build/config/android/config.gni")
 }
 
 # TODO(sky): nuke this. Temporary while sorting out http://crbug.com/465456.
 enable_correct_v8_arch = false
 
-v8_use_external_startup_data = !is_ios
-
 # Turns on compiler optimizations in V8 in Debug build.
 v8_optimized_debug = true
 
 # V8 extras
 # Adding V8 extras files requires API owners review
 # Be sure to synchronize with build/common.gypi
 
 v8_extra_library_files =
     [ "../third_party/WebKit/Source/core/streams/ReadableStreamTempStub.js" ]
 v8_experimental_extra_library_files = [
@@ skipping 12 matching lines @@
       "natives_blob_32.bin",
       "snapshot_blob_32.bin",
     ]
   } else {
     v8_external_startup_data_renaming_destinations = [
       "natives_blob_64.bin",
       "snapshot_blob_64.bin",
     ]
   }
 }