ObjPtr<>-ify mirror::Object::As*(), fix stale refs in test.

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Bug: 31113334
Change-Id: Ia04c4996f5cbfa1f26acbadcd747342c57b1aac3
diff --git a/runtime/runtime_callbacks_test.cc b/runtime/runtime_callbacks_test.cc
index df06a9f..a4f3df2 100644
--- a/runtime/runtime_callbacks_test.cc
+++ b/runtime/runtime_callbacks_test.cc
@@ -448,14 +448,14 @@
         return false;
       }
       std::lock_guard<std::mutex> lock(ref_guard_);
-      mirror::Class* k = obj->AsClass();
+      ObjPtr<mirror::Class> k = obj->AsClass();
       ClassReference test = { &k->GetDexFile(), k->GetDexClassDefIndex() };
       return ref_ == test;
     }
 
     void SetInterestingObject(mirror::Object* obj) REQUIRES_SHARED(art::Locks::mutator_lock_) {
       std::lock_guard<std::mutex> lock(ref_guard_);
-      mirror::Class* k = obj->AsClass();
+      ObjPtr<mirror::Class> k = obj->AsClass();
       ref_ = { &k->GetDexFile(), k->GetDexClassDefIndex() };
     }