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 4c2b479182db68654866d2fbb128dff6987548ed..e1ba80176029df840ddea891d78905faa6bc36ba 100644
--- a/src/json-parser.h
+++ b/src/json-parser.h
@@ -414,9 +414,8 @@ Handle<Object> JsonParser<seq_ascii>::ParseJsonObject() {
           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 = factory()->NewMutableHeapNumber(value->Number());
             }
             properties.Add(value, zone());
             map = target;