[gn] Add swarming support

gni/v8.gni

 # Copyright 2016 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/sanitizers/sanitizers.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
+
+  # Use external files for startup data blobs:
+  # the JS builtins sources and the start snapshot.
+  v8_use_external_startup_data = !is_ios
+
   # V8 generates code for this architecture. If v8_target_arch differs from
   # target_cpu, a simulator will be run.
   v8_target_arch = ""
 }
 
 if (v8_target_arch == "") {
   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
   }
 }