Move mirror::ObjectArray to ObjPtr

Also reduce calls to ObjPtr::Ptr.

Bug: 31113334

Test: test-art-host -j32

Change-Id: I73e1b356972bb079b66332f00f7f07451601044e
diff --git a/runtime/mirror/object_reference.h b/runtime/mirror/object_reference.h
index 573cb30..71f34c6 100644
--- a/runtime/mirror/object_reference.h
+++ b/runtime/mirror/object_reference.h
@@ -42,6 +42,9 @@
     reference_ = Compress(other);
   }
 
+  void Assign(ObjPtr<MirrorType> ptr)
+      REQUIRES_SHARED(Locks::mutator_lock_);
+
   void Clear() {
     reference_ = 0;
     DCHECK(IsNull());