[serializer] reserve maps one by one to avoid fragmentation.

src/snapshot/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_SERIALIZER_H_
 #define V8_SNAPSHOT_SERIALIZER_H_
 
 #include "src/isolate.h"
 #include "src/log.h"
 #include "src/objects.h"
@@ skipping 172 matching lines @@
     if (skip != 0) {
       sink_.Put(kSkip, "SkipFromSerializeObject");
       sink_.PutInt(skip, "SkipDistanceFromSerializeObject");
     }
   }
 
   bool BackReferenceIsAlreadyAllocated(SerializerReference back_reference);
 
   // This will return the space for an object.
   SerializerReference AllocateLargeObject(int size);
+  SerializerReference AllocateMap();
   SerializerReference Allocate(AllocationSpace space, int size);
   int EncodeExternalReference(Address addr) {
     return external_reference_encoder_.Encode(addr);
   }
 
   bool HasNotExceededFirstPageOfEachSpace();
 
   // GetInt reads 4 bytes at once, requiring padding at the end.
   void Pad();
 
@@ skipping 35 matching lines @@
 
  private:
   CodeAddressMap* code_address_map_;
   // Objects from the same space are put into chunks for bulk-allocation
   // when deserializing. We have to make sure that each chunk fits into a
   // page. So we track the chunk size in pending_chunk_ of a space, but
   // when it exceeds a page, we complete the current chunk and start a new one.
   uint32_t pending_chunk_[kNumberOfPreallocatedSpaces];
   List<uint32_t> completed_chunks_[kNumberOfPreallocatedSpaces];
   uint32_t max_chunk_size_[kNumberOfPreallocatedSpaces];
+  // Number of maps that we need to allocate.
+  uint32_t num_maps_;
 
   // We map serialized large objects to indexes for back-referencing.
   uint32_t large_objects_total_size_;
   uint32_t seen_large_objects_index_;
 
   List<byte> code_buffer_;
 
   // To handle stack overflow.
   List<HeapObject*> deferred_objects_;
 
@@ skipping 62 matching lines @@
   SnapshotByteSink* sink_;
   int reference_representation_;
   int bytes_processed_so_far_;
   bool code_has_been_output_;
 };
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_SNAPSHOT_SERIALIZER_H_