Move mirror::ObjectArray to ObjPtr

Also reduce calls to ObjPtr::Ptr.

Bug: 31113334

Test: test-art-host -j32

Change-Id: I73e1b356972bb079b66332f00f7f07451601044e
diff --git a/runtime/jni_internal.cc b/runtime/jni_internal.cc
index 6828124..8eebe56 100644
--- a/runtime/jni_internal.cc
+++ b/runtime/jni_internal.cc
@@ -379,7 +379,7 @@
       // Not even a java.lang.reflect.Field, return null. TODO, is this check necessary?
       return nullptr;
     }
-    ObjPtr<mirror::Field> field = down_cast<mirror::Field*>(obj_field.Ptr());
+    ObjPtr<mirror::Field> field = ObjPtr<mirror::Field>::DownCast(obj_field);
     return soa.EncodeField(field->GetArtField());
   }