Add Core methods for wait set.

mojo/edk/system/wait_set_dispatcher.cc

Index: mojo/edk/system/wait_set_dispatcher.cc
diff --git a/mojo/edk/system/wait_set_dispatcher.cc b/mojo/edk/system/wait_set_dispatcher.cc
index a807ce2e871bbd910973e8b577d8b785e1d8189d..fdf420abe4cb34bb16217f08690f271336d8b3dd 100644
--- a/mojo/edk/system/wait_set_dispatcher.cc
+++ b/mojo/edk/system/wait_set_dispatcher.cc
@@ -152,10 +152,10 @@ WaitSetDispatcher::CreateEquivalentDispatcherAndCloseImplNoLock(
 }
 
 MojoResult WaitSetDispatcher::WaitSetAddImpl(
-    UserPointer<const MojoWaitSetAddOptions> options,
     RefPtr<Dispatcher>&& dispatcher,
     MojoHandleSignals signals,
-    uint64_t cookie) {
+    uint64_t cookie,
+    UserPointer<const MojoWaitSetAddOptions> options) {
   // This will be owned by |entries_|, so it should only be used under
   // |mutex()|.
   Entry* entry = nullptr;