Replace HeapNumber as doublebox with an explicit MutableHeapNumber.

src/json-parser.h

Index: src/json-parser.h
diff --git a/src/json-parser.h b/src/json-parser.h
index 09320ab2bddfe3ae1ff84e35ce776ca00ea1f8fb..cdbb50d6e88c3281b6a7fd83b1935f3c828cfe26 100644
--- a/src/json-parser.h
+++ b/src/json-parser.h
@@ -387,11 +387,9 @@ Handle<Object> JsonParser<seq_ascii>::ParseJsonObject() {
           Representation expected_representation = details.representation();
 
           if (value->FitsRepresentation(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());
+            if (expected_representation.IsDouble()) {
+              value = Object::NewStorageFor(isolate(), value,
+                                            expected_representation);
             } else if (expected_representation.IsHeapObject() &&
                        !target->instance_descriptors()->GetFieldType(
                            descriptor)->NowContains(value)) {