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

mojo/edk/system/wait_set_dispatcher.cc

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "mojo/edk/system/wait_set_dispatcher.h"
 
 #include "base/logging.h"
 #include "mojo/edk/system/options_validation.h"
 
 using mojo::util::MutexLocker;
@@ skipping 54 matching lines @@
 
 RefPtr<Dispatcher>
 WaitSetDispatcher::CreateEquivalentDispatcherAndCloseImplNoLock(
     MessagePipe* /*message_pipe*/,
     unsigned /*port*/) {
   mutex().AssertHeld();
   NOTREACHED();
   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();
   return MOJO_RESULT_UNIMPLEMENTED;
 }
 
 MojoResult WaitSetDispatcher::WaitSetWaitImpl(
     MojoDeadline deadline,
     UserPointer<uint32_t> num_results,
     UserPointer<MojoWaitSetResult> results,
     UserPointer<uint32_t> max_results) {
   MutexLocker locker(&mutex());
   if (is_closed_no_lock())
     return MOJO_RESULT_INVALID_ARGUMENT;
 
   // TODO(vtl)
   NOTIMPLEMENTED();
   return MOJO_RESULT_UNIMPLEMENTED;
 }
 
 }  // namespace system
 }  // namespace mojo