Don't call the other wait set impl dispatcher methods under mutex either.

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 fa8a56ca83d36b1b3341b12c9b98aaa1c878db06..1626f2ff24f5b2106aa7a1d1a022dff2d7b24fbe 100644
--- a/mojo/edk/system/wait_set_dispatcher.cc
+++ b/mojo/edk/system/wait_set_dispatcher.cc
@@ -72,20 +72,24 @@ WaitSetDispatcher::CreateEquivalentDispatcherAndCloseImplNoLock(
   return nullptr;
 }
 
-MojoResult WaitSetDispatcher::WaitSetAddImplNoLock(
+MojoResult WaitSetDispatcher::WaitSetAddImpl(
     UserPointer<const MojoWaitSetAddOptions> options,
     Handle&& handle,
     MojoHandleSignals signals,
     uint64_t cookie) {
-  mutex().AssertHeld();
+  MutexLocker locker(&mutex());
+  if (is_closed_no_lock())
+    return MOJO_RESULT_INVALID_ARGUMENT;
 
   // TODO(vtl)
   NOTIMPLEMENTED();
   return MOJO_RESULT_UNIMPLEMENTED;
 }
 
-MojoResult WaitSetDispatcher::WaitSetRemoveImplNoLock(uint64_t cookie) {
-  mutex().AssertHeld();
+MojoResult WaitSetDispatcher::WaitSetRemoveImpl(uint64_t cookie) {
+  MutexLocker locker(&mutex());
+  if (is_closed_no_lock())
+    return MOJO_RESULT_INVALID_ARGUMENT;
 
   // TODO(vtl)
   NOTIMPLEMENTED();