Support external startup data in V8.

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index 2caf0c0fc391a3f05ba65f7f8889acbb3dca1606..7d50f766b5c07b2027f31acb12201e59cc13fa2a 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -16,6 +16,7 @@ v8_object_print = false
 v8_postmortem_support = false
 v8_use_default_platform = true
 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"
@@ -97,6 +98,11 @@ config("features") {
       "ENABLE_HANDLE_ZAPPING",
     ]
   }
+  if (v8_use_external_startup_data == true) {
+    defines += [
+      "V8_USE_EXTERNAL_STARTUP_DATA",
+    ]
+  }
 }
 
 config("toolchain") {
@@ -303,6 +309,7 @@ source_set("v8_nosnapshot") {
     "$target_gen_dir/experimental-libraries.cc",
     "$target_gen_dir/trig-table.cc",
     "src/snapshot-empty.cc",
+    "src/snapshot-common.cc",
   ]
 
   configs -= [ "//build/config/compiler:chromium_code" ]
@@ -636,7 +643,8 @@ source_set("v8_base") {
     "src/serialize.h",
     "src/small-pointer-list.h",
     "src/smart-pointers.h",
-    "src/snapshot-common.cc",
+    "src/snapshot-source-sink.cc",
+    "src/snapshot-source-sink.h",
     "src/snapshot.h",
     "src/spaces-inl.h",
     "src/spaces.cc",