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

src/snapshot/serializer-common.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_COMMON_H_
 #define V8_SNAPSHOT_SERIALIZER_COMMON_H_
 
 #include "src/address-map.h"
 #include "src/external-reference-table.h"
 #include "src/globals.h"
@@ skipping 62 matching lines @@
 };
 
 // The Serializer/Deserializer class is a common superclass for Serializer and
 // Deserializer which is used to store common constants and methods used by
 // both.
 class SerializerDeserializer : public ObjectVisitor {
  public:
   static void Iterate(Isolate* isolate, ObjectVisitor* visitor);
 
   // No reservation for large object space necessary.
-  static const int kNumberOfPreallocatedSpaces = LAST_PAGED_SPACE + 1;
+  // We also handle map space differenly.
+  STATIC_ASSERT(MAP_SPACE == CODE_SPACE + 1);
+  static const int kNumberOfPreallocatedSpaces = CODE_SPACE + 1;
   static const int kNumberOfSpaces = LAST_SPACE + 1;
 
  protected:
   static bool CanBeDeferred(HeapObject* o);
 
   // ---------- byte code range 0x00..0x7f ----------
   // Byte codes in this range represent Where, HowToCode and WhereToPoint.
   // Where the pointed-to object can be found:
   // The static assert below will trigger when the number of preallocated spaces
   // changed. If that happens, update the bytecode ranges in the comments below.
@@ skipping 182 matching lines @@
 
   byte* data_;
   int size_;
   bool owns_data_;
 };
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_SNAPSHOT_SERIALIZER_COMMON_H_