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

BUG=421063

Review URL: https://codereview.chromium.org/749213004

Cr-Commit-Position: refs/heads/master@{#25691}

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_deprecation_warnings = false
@@ -14,7 +18,6 @@ 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"
@@ -1400,7 +1403,7 @@ 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",
@@ -1411,6 +1414,7 @@ component("v8") {
       ":v8_snapshot",
     ]
   } else {
+    assert(!v8_use_external_startup_data)
     deps = [
       ":v8_base",
       ":v8_nosnapshot",
@@ -1432,7 +1436,7 @@ 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",
@@ -1443,6 +1447,7 @@ group("v8") {
       ":v8_snapshot",
     ]
   } else {
+    assert(!v8_use_external_startup_data)
     deps = [
       ":v8_base",
       ":v8_nosnapshot",