[runtime] Add MapUpdater class that manages all kinds of map updates.

src/json-parser.cc

Index: src/json-parser.cc
diff --git a/src/json-parser.cc b/src/json-parser.cc
index 5e79b611a2f787c3e670829817dced6a73e8e731..78b325daa6a7b17bbac3afd5c80e086c4431cf04 100644
--- a/src/json-parser.cc
+++ b/src/json-parser.cc
@@ -399,8 +399,8 @@ Handle<Object> JsonParser<seq_one_byte>::ParseJsonObject() {
                    ->NowContains(value)) {
             Handle<FieldType> value_type(
                 value->OptimalType(isolate(), expected_representation));
-            Map::GeneralizeFieldType(target, descriptor,
-                                     expected_representation, value_type);
+            Map::GeneralizeField(target, descriptor, expected_representation,
+                                 value_type);
           }
           DCHECK(target->instance_descriptors()
                      ->GetFieldType(descriptor)