Add ClassExt mirror.

We add the ClassExt type to hold values in Class that are not always
needed without making the Class type larger. For the initial change we
move the verifyError field into this type. It is expected to grow as
time goes on.

Test: mma test-art-host

Change-Id: I1d97df2e1267203841ad47b2effcb66dd76ac12a
diff --git a/runtime/well_known_classes.cc b/runtime/well_known_classes.cc
index 2797d85..53d717a 100644
--- a/runtime/well_known_classes.cc
+++ b/runtime/well_known_classes.cc
@@ -393,7 +393,9 @@
 }
 
 ObjPtr<mirror::Class> WellKnownClasses::ToClass(jclass global_jclass) {
-  return ObjPtr<mirror::Class>::DownCast(Thread::Current()->DecodeJObject(global_jclass));
+  auto ret = ObjPtr<mirror::Class>::DownCast(Thread::Current()->DecodeJObject(global_jclass));
+  DCHECK(!ret.IsNull());
+  return ret;
 }
 
 }  // namespace art