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/well_known_classes.h b/runtime/well_known_classes.h
index ddfc5b8..248ba7f 100644
--- a/runtime/well_known_classes.h
+++ b/runtime/well_known_classes.h
@@ -19,6 +19,7 @@
 
 #include "base/mutex.h"
 #include "jni.h"
+#include "obj_ptr.h"
 
 namespace art {
 
@@ -41,8 +42,7 @@
   static ArtMethod* StringInitToStringFactory(ArtMethod* method);
   static uint32_t StringInitToEntryPoint(ArtMethod* method);
 
-  static mirror::Class* ToClass(jclass global_jclass)
-      REQUIRES_SHARED(Locks::mutator_lock_);
+  static ObjPtr<mirror::Class> ToClass(jclass global_jclass) REQUIRES_SHARED(Locks::mutator_lock_);
 
   static jclass com_android_dex_Dex;
   static jclass dalvik_annotation_optimization_CriticalNative;