Add different behavior to AwakableList for "persistent" vs "one-shot" awakables.

mojo/edk/system/async_waiter.cc

 // Copyright 2014 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/async_waiter.h"
 
 namespace mojo {
 namespace system {
 
 AsyncWaiter::AsyncWaiter(const AwakeCallback& callback) : callback_(callback) {}
 
 AsyncWaiter::~AsyncWaiter() {}
 
-bool AsyncWaiter::Awake(uint64_t /*context*/,
+void AsyncWaiter::Awake(uint64_t /*context*/,
                         AwakeReason reason,
                         const HandleSignalsState& /*signals_state*/) {
   callback_(MojoResultForAwakeReason(reason));
   delete this;
-  return false;
 }
 
 }  // namespace system
 }  // namespace mojo