[headless] Use embedded v8 data for headless.

build_overrides/v8.gni

Index: build_overrides/v8.gni
diff --git a/build_overrides/v8.gni b/build_overrides/v8.gni
index c39f47e3b00f082971a63e8488e3466a447c436b..f5330cf6e4ce0c907a3f59f5311d674567fc39e0 100644
--- a/build_overrides/v8.gni
+++ b/build_overrides/v8.gni
@@ -2,6 +2,10 @@
 # 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")
 }
@@ -9,8 +13,6 @@ if (is_android) {
 # 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