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

src/snapshot/deserializer.cc

 // 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.
 
 #include "src/snapshot/deserializer.h"
 
 #include "src/bootstrapper.h"
 #include "src/heap/heap.h"
 #include "src/isolate.h"
 #include "src/macro-assembler.h"
@@ skipping 59 matching lines @@
            SerializedData::ComputeMagicNumber(external_reference_table_));
 }
 
 void Deserializer::Deserialize(Isolate* isolate) {
   Initialize(isolate);
   if (!ReserveSpace()) V8::FatalProcessOutOfMemory("deserializing context");
   // No active threads.
   DCHECK_NULL(isolate_->thread_manager()->FirstThreadStateInUse());
   // No active handles.
   DCHECK(isolate_->handle_scope_implementer()->blocks()->is_empty());
+  // Partial snapshot cache is not yet populated.
+  DCHECK(isolate_->partial_snapshot_cache()->is_empty());
 
   {
     DisallowHeapAllocation no_gc;
     isolate_->heap()->IterateSmiRoots(this);
     isolate_->heap()->IterateStrongRoots(this, VISIT_ONLY_STRONG);
     isolate_->heap()->RepairFreeListsAfterDeserialization();
     isolate_->heap()->IterateWeakRoots(this, VISIT_ALL);
     DeserializeDeferredObjects();
     FlushICacheForNewIsolate();
   }
@@ skipping 718 matching lines @@
 
       default:
         CHECK(false);
     }
   }
   CHECK_EQ(limit, current);
   return true;
 }
 }  // namespace internal
 }  // namespace v8