[serializer] Add API to warm up startup snapshot with an additional script.

src/isolate.h

Index: src/isolate.h
diff --git a/src/isolate.h b/src/isolate.h
index c51caf3212529b3a593b456dca8a88b436662f9f..e0147cb59c62f07be12de7105660e80bffcea947 100644
--- a/src/isolate.h
+++ b/src/isolate.h
@@ -1358,6 +1358,8 @@ class Isolate {
   friend class v8::Locker;
   friend class v8::Unlocker;
   friend v8::StartupData v8::V8::CreateSnapshotDataBlob(const char*);
+  friend v8::StartupData v8::V8::WarmUpSnapshotDataBlob(v8::StartupData,
+                                                        const char*);
 
   DISALLOW_COPY_AND_ASSIGN(Isolate);
 };