make v8_use_snapshot configurable in gn

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
 # Chromium build.
 import("//build_overrides/v8.gni")
 
 import("snapshot_toolchain.gni")
 
+declare_args() {
+  # Enable the snapshot feature, for fast context creation.
+  # http://v8project.blogspot.com/2015/09/custom-startup-snapshots.html
+  v8_use_snapshot = true
+}
+
 # TODO(jochen): These will need to be user-settable to support standalone V8
 # builds.
 v8_deprecation_warnings = false
 v8_enable_disassembler = false
 v8_enable_gdbjit = false
 v8_enable_handle_zapping = true
 v8_enable_i18n_support = true
 v8_enable_verify_heap = false
 v8_interpreted_regexp = false
 v8_object_print = false
 v8_postmortem_support = false
-v8_use_snapshot = true
 v8_random_seed = "314159265"
 v8_toolset_for_d8 = "host"
 
 if (is_msan) {
   # Running the V8-generated code on an ARM simulator is a powerful hack that
   # allows the tool to see the memory accesses from JITted code. Without this
   # flag, JS code causes false positive reports from MSan.
   v8_target_arch = "arm64"
 } else {
   v8_target_arch = target_cpu
@@ skipping 1802 matching lines @@
     if (!is_component_build) {
       sources += [
         "$target_gen_dir/d8-js.cc",
       ]
     }
     if (v8_enable_i18n_support) {
       deps += [ "//third_party/icu" ]
     }
   }
 }