ObjPtr<>-ify method/var handles, fix stale refs in tests.

Test: m test-art-host-gtest
Test: testrunner.py --host --interpreter
Bug: 31113334
Change-Id: I89ea84f7970899643e437161b598d3232a182e98
diff --git a/runtime/method_handles_test.cc b/runtime/method_handles_test.cc
index 6a7eb8c..688febc 100644
--- a/runtime/method_handles_test.cc
+++ b/runtime/method_handles_test.cc
@@ -46,8 +46,8 @@
     return throwable->GetClass()->DescriptorEquals("Ljava/lang/invoke/WrongMethodTypeException;");
   }
 
-  static mirror::MethodType* CreateVoidMethodType(Thread* self,
-                                                  Handle<mirror::Class> parameter_type)
+  static ObjPtr<mirror::MethodType> CreateVoidMethodType(Thread* self,
+                                                         Handle<mirror::Class> parameter_type)
         REQUIRES_SHARED(Locks::mutator_lock_) {
     ClassLinker* cl = Runtime::Current()->GetClassLinker();
     StackHandleScope<2> hs(self);