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

src/snapshot/startup-serializer.h

 // Copyright 2016 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_SNAPSHOT_STARTUP_SERIALIZER_H_
 #define V8_SNAPSHOT_STARTUP_SERIALIZER_H_
 
 #include "src/snapshot/serializer.h"
 
 namespace v8 {
 namespace internal {
 
 class StartupSerializer : public Serializer {
  public:
-  StartupSerializer(Isolate* isolate, SnapshotByteSink* sink);
+  enum FunctionCodeHandling { CLEAR_FUNCTION_CODE, KEEP_FUNCTION_CODE };
+
+  StartupSerializer(
+      Isolate* isolate, SnapshotByteSink* sink,
+      FunctionCodeHandling function_code_handling = CLEAR_FUNCTION_CODE);
   ~StartupSerializer() override;
 
   // Serialize the current state of the heap.  The order is:
   // 1) Strong references.
   // 2) Partial snapshot cache.
   // 3) Weak references (e.g. the string table).
   void SerializeStrongReferences();
   void SerializeWeakReferencesAndDeferred();
 
  private:
   // The StartupSerializer has to serialize the root array, which is slightly
   // different.
   void VisitPointers(Object** start, Object** end) override;
   void SerializeObject(HeapObject* o, HowToCode how_to_code,
                        WhereToPoint where_to_point, int skip) override;
   void Synchronize(VisitorSynchronization::SyncTag tag) override;
 
   intptr_t root_index_wave_front_;
   bool serializing_builtins_;
+  FunctionCodeHandling function_code_handling_;
   DISALLOW_COPY_AND_ASSIGN(StartupSerializer);
 };
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_SNAPSHOT_STARTUP_SERIALIZER_H_