Revert "Replace HeapNumber as doublebox with an explicit MutableHeapNumber."

src/json-parser.h

 // Copyright 2011 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_JSON_PARSER_H_
 #define V8_JSON_PARSER_H_
 
 #include "src/v8.h"
 
 #include "src/char-predicates-inl.h"
@@ skipping 369 matching lines @@
         value = ParseJsonValue();
         if (value.is_null()) return ReportUnexpectedCharacter();
 
         if (transitioning) {
           int descriptor = map->NumberOfOwnDescriptors();
           PropertyDetails details =
               target->instance_descriptors()->GetDetails(descriptor);
           Representation expected_representation = details.representation();
 
           if (value->FitsRepresentation(expected_representation)) {
-            if (expected_representation.IsDouble()) {
-              value = Object::NewStorageFor(isolate(), value,
-                                            expected_representation);
+            // If the target representation is double and the value is already
+            // double, use the existing box.
+            if (value->IsSmi() && expected_representation.IsDouble()) {
+              value = factory()->NewHeapNumber(
+                  Handle<Smi>::cast(value)->value());
             } else if (expected_representation.IsHeapObject() &&
                        !target->instance_descriptors()->GetFieldType(
                            descriptor)->NowContains(value)) {
               Handle<HeapType> value_type(value->OptimalType(
                       isolate(), expected_representation));
               Map::GeneralizeFieldType(target, descriptor, value_type);
             }
             ASSERT(target->instance_descriptors()->GetFieldType(
                     descriptor)->NowContains(value));
             properties.Add(value, zone());
@@ skipping 378 matching lines @@
 
   ASSERT_EQ('"', c0_);
   // Advance past the last '"'.
   AdvanceSkipWhitespace();
   return result;
 }
 
 } }  // namespace v8::internal
 
 #endif  // V8_JSON_PARSER_H_