GN: Enable embedder to decide whether or not the external V8 snapshot is enabled.

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index 1c2c52a97ec2294c88c152da6f7e578da8a2204e..2f713ca6a4c3e64c9d8b2ee98ac9665f9bb97c65 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -2,6 +2,10 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+# Because standalone V8 builds are not supported, assume this is part of a
+# Chromium build.
+import("//build/module_args/v8.gni")
+
 # TODO(jochen): These will need to be user-settable to support standalone V8
 # builds.
 v8_compress_startup_data = "off"
@@ -15,7 +19,6 @@ v8_interpreted_regexp = false
 v8_object_print = false
 v8_postmortem_support = false
 v8_use_snapshot = true
-v8_use_external_startup_data = false
 v8_enable_extra_checks = is_debug
 v8_target_arch = cpu_arch
 v8_random_seed = "314159265"
@@ -1412,7 +1415,7 @@ component("v8") {
     "src/v8dll-main.cc",
   ]
 
-  if (v8_use_external_startup_data) {
+  if (v8_use_snapshot && v8_use_external_startup_data) {
     deps = [
       ":v8_base",
       ":v8_external_snapshot",
@@ -1423,6 +1426,7 @@ component("v8") {
       ":v8_snapshot",
     ]
   } else {
+    assert(!v8_use_external_startup_data)
     deps = [
       ":v8_base",
       ":v8_nosnapshot",
@@ -1444,7 +1448,7 @@ component("v8") {
 } else {
 
 group("v8") {
-  if (v8_use_external_startup_data) {
+  if (v8_use_snapshot && v8_use_external_startup_data) {
     deps = [
       ":v8_base",
       ":v8_external_snapshot",
@@ -1455,6 +1459,7 @@ group("v8") {
       ":v8_snapshot",
     ]
   } else {
+    assert(!v8_use_external_startup_data)
     deps = [
       ":v8_base",
       ":v8_nosnapshot",