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

mojo/edk/system/async_waiter.h

 // 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.
 
 #ifndef MOJO_EDK_SYSTEM_ASYNC_WAITER_H_
 #define MOJO_EDK_SYSTEM_ASYNC_WAITER_H_
 
 #include <functional>
 
 #include "mojo/edk/system/awakable.h"
 #include "mojo/public/c/system/result.h"
 #include "mojo/public/cpp/system/macros.h"
 
 namespace mojo {
 namespace system {
 
-// An |Awakable| implementation that just calls a given callback object.
+// An |Awakable| implementation that just calls a given callback object. It
+// should be used in a non-persistent way (i.e., |Awake()| should be called at
+// most once by each source, and only for "leading edges").
 class AsyncWaiter final : public Awakable {
  public:
   using AwakeCallback = std::function<void(MojoResult)>;
 
   // |callback| must satisfy the same contract as |Awakable::Awake()|.
   explicit AsyncWaiter(const AwakeCallback& callback);
   ~AsyncWaiter() override;
 
  private:
   // |Awakable| implementation:
-  bool Awake(uint64_t context,
+  void Awake(uint64_t context,
              AwakeReason reason,
              const HandleSignalsState& signals_state) override;
 
   AwakeCallback callback_;
 
   MOJO_DISALLOW_COPY_AND_ASSIGN(AsyncWaiter);
 };
 
 }  // namespace system
 }  // namespace mojo
 
 #endif  // MOJO_EDK_SYSTEM_ASYNC_WAITER_H_