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

src/snapshot/deserializer.h

Index: src/snapshot/deserializer.h
diff --git a/src/snapshot/deserializer.h b/src/snapshot/deserializer.h
index 3f66708e56f6c76779c9bf264b3e6c336e40fdfe..3539fe8e088617a9b0dd646be61732ef2bc1fefd 100644
--- a/src/snapshot/deserializer.h
+++ b/src/snapshot/deserializer.h
@@ -34,6 +34,7 @@ class Deserializer : public SerializerDeserializer {
       : isolate_(NULL),
         source_(data->Payload()),
         magic_number_(data->GetMagicNumber()),
+        next_map_index_(0),
         external_reference_table_(NULL),
         deserialized_large_objects_(0),
         deserializing_user_code_(false),
@@ -129,6 +130,8 @@ class Deserializer : public SerializerDeserializer {
   Heap::Reservation reservations_[kNumberOfSpaces];
   uint32_t current_chunk_[kNumberOfPreallocatedSpaces];
   Address high_water_[kNumberOfPreallocatedSpaces];
+  int next_map_index_;
+  List<Address> allocated_maps_;
 
   ExternalReferenceTable* external_reference_table_;