layers: update valid usage

Change-Id: Id7556ddc1be59ee7fe5ab9273257cbc399724a3e
diff --git a/layers/object_tracker.cpp b/layers/object_tracker.cpp
index acd518a..4408c14 100644
--- a/layers/object_tracker.cpp
+++ b/layers/object_tracker.cpp
@@ -363,6 +363,7 @@
         }
     }
 
+    // TODO: The instance handle can not be validated here. The loader will likely have to validate it.
     ValidateObject(instance, instance, VK_DEBUG_REPORT_OBJECT_TYPE_INSTANCE_EXT, true, VALIDATION_ERROR_00021);
 
     DestroyObject(instance, instance, VK_DEBUG_REPORT_OBJECT_TYPE_INSTANCE_EXT, pAllocator);