Move remaining jobject related functions to use ObjPtr

Also added ObjPtr::DownCast.

Bug: 31113334

Test: test-art-host

Change-Id: I59c253211dc435579ffdfd49f856861ab13d262c
diff --git a/runtime/openjdkjvmti/transform.cc b/runtime/openjdkjvmti/transform.cc
index f59e01e..3443aea 100644
--- a/runtime/openjdkjvmti/transform.cc
+++ b/runtime/openjdkjvmti/transform.cc
@@ -327,8 +327,7 @@
         class_linker->FindClass(self, dex_file_name, null_loader)
           ->FindDeclaredInstanceField("mInternalCookie", "Ljava/lang/Object;");
     CHECK(dex_file_cookie_field != nullptr);
-    art::Handle<art::mirror::Class> klass(
-        hs.NewHandle(art::down_cast<art::mirror::Class*>(self->DecodeJObject(jklass))));
+    art::Handle<art::mirror::Class> klass(hs.NewHandle(self->DecodeJObject(jklass)->AsClass()));
     art::mirror::Object* dex_file_ptr = nullptr;
     art::mirror::ClassLoader* class_loader_ptr = nullptr;
     // Find dalvik.system.DexFile that represents the dex file we are changing.