Address some code comments

Change-Id: I0a38a387c4328d45bbc04d095bf3388c27495c12
diff --git a/runtime/class_linker.h b/runtime/class_linker.h
index f705330..739403f 100644
--- a/runtime/class_linker.h
+++ b/runtime/class_linker.h
@@ -560,7 +560,7 @@
 
  private:
   struct ClassLoaderData {
-    jobject weak_root;  // Weak root to enable class unloading.
+    jweak weak_root;  // Weak root to enable class unloading.
     ClassTable* class_table;
     LinearAlloc* allocator;
   };