Partially ObjPtr<>-ify HandleScope.

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing --interpreter
Bug: 31113334
Change-Id: I560683b5345bda9df4b22903d032b593d6c2dd4f
diff --git a/runtime/stack.h b/runtime/stack.h
index 4bc0fc8..ff80d13 100644
--- a/runtime/stack.h
+++ b/runtime/stack.h
@@ -194,7 +194,7 @@
 
   uint32_t GetDexPc(bool abort_on_failure = true) const REQUIRES_SHARED(Locks::mutator_lock_);
 
-  mirror::Object* GetThisObject() const REQUIRES_SHARED(Locks::mutator_lock_);
+  ObjPtr<mirror::Object> GetThisObject() const REQUIRES_SHARED(Locks::mutator_lock_);
 
   size_t GetNativePcOffset() const REQUIRES_SHARED(Locks::mutator_lock_);