Support external startup data in V8.

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index 2420ecfbddb467b3a91f44d0c4422749957dc49e..ec5d8dcff6a8b21e8942721d9a9abc0abeac9499 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
 
 if (is_debug) {
   v8_enable_extra_checks = true
@@ -103,6 +104,11 @@ config("features") {
       "ENABLE_HANDLE_ZAPPING",
     ]
   }
+  if (v8_use_external_startup_data == true) {
+    defines += [
+      "V8_USE_EXTERNAL_STARTUP_DATA",
+    ]
+  }
 }
 
 ###############################################################################
@@ -237,6 +243,7 @@ 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" ]
@@ -545,7 +552,8 @@ 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",