Address some comments and clean up

Change-Id: I538cf204f1c89d5fc81f8fc5e5800fcf1cf87359
diff --git a/runtime/gc/weak_root_state.h b/runtime/gc/weak_root_state.h
index b66f19d..e3cefc4 100644
--- a/runtime/gc/weak_root_state.h
+++ b/runtime/gc/weak_root_state.h
@@ -28,6 +28,8 @@
   // Need to wait until we can read weak roots.
   kWeakRootStateNoReadsOrWrites,
   // Need to mark new weak roots to make sure they don't get swept.
+  // kWeakRootStateMarkNewRoots is currently unused but I was planning on using to allow adding new
+  // weak roots during the CMS reference processing phase.
   kWeakRootStateMarkNewRoots,
 };