Add options validation code in WaitSetDispatcher::WaitSetAddImpl().

mojo/edk/system/wait_set_dispatcher.h

Index: mojo/edk/system/wait_set_dispatcher.h
diff --git a/mojo/edk/system/wait_set_dispatcher.h b/mojo/edk/system/wait_set_dispatcher.h
index b749d576cf2d505ee142b053092e8bc6cd3c26da..56e8654b9e54a65985c711ce81de1300346c80d5 100644
--- a/mojo/edk/system/wait_set_dispatcher.h
+++ b/mojo/edk/system/wait_set_dispatcher.h
@@ -37,6 +37,11 @@ class WaitSetDispatcher final : public Dispatcher {
       UserPointer<const MojoCreateWaitSetOptions> in_options,
       MojoCreateWaitSetOptions* out_options);
 
+  // Like |ValidateCreateOptions()|, but for |MojoWaitSetAddOptions|.
+  static MojoResult ValidateWaitSetAddOptions(
+      UserPointer<const MojoWaitSetAddOptions> in_options,
+      MojoWaitSetAddOptions* out_options);
+
   static util::RefPtr<WaitSetDispatcher> Create(
       const MojoCreateWaitSetOptions& /*validated_options*/) {
     return AdoptRef(new WaitSetDispatcher());