Give AwakableList's wake-up method the old state in addition to the new state.

mojo/edk/system/simple_dispatcher.h

 // Copyright 2013 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_SIMPLE_DISPATCHER_H_
 #define MOJO_EDK_SYSTEM_SIMPLE_DISPATCHER_H_
 
 #include <list>
 
 #include "mojo/edk/system/awakable_list.h"
 #include "mojo/edk/system/dispatcher.h"
+#include "mojo/edk/system/handle_signals_state.h"
 #include "mojo/edk/util/thread_annotations.h"
 #include "mojo/public/cpp/system/macros.h"
 
 namespace mojo {
 namespace system {
 
 // A base class for simple dispatchers. "Simple" means that there's a one-to-one
 // correspondence between handles and dispatchers (see the explanatory comment
 // in core.cc). This class implements the standard waiter-signaling mechanism in
 // that case.
 class SimpleDispatcher : public Dispatcher {
  protected:
   SimpleDispatcher();
   ~SimpleDispatcher() override;
 
-  // To be called by subclasses when the state changes (so
-  // |GetHandleSignalsStateImplNoLock()| should be checked again).
-  void HandleSignalsStateChangedNoLock() MOJO_EXCLUSIVE_LOCKS_REQUIRED(mutex());
+  // To be called by subclasses when the state changes.
+  void OnHandleSignalsStateChangeNoLock(const HandleSignalsState& old_state,
+                                        const HandleSignalsState& new_state)
+      MOJO_EXCLUSIVE_LOCKS_REQUIRED(mutex());
 
   // |Dispatcher| protected methods:
   void CancelAllStateNoLock() override;
   MojoResult AddAwakableImplNoLock(Awakable* awakable,
                                    MojoHandleSignals signals,
                                    bool force,
                                    uint64_t context,
                                    HandleSignalsState* signals_state) override;
   void RemoveAwakableImplNoLock(Awakable* awakable,
                                 HandleSignalsState* signals_state) override;
   void RemoveAwakableWithContextImplNoLock(
       Awakable* awakable,
       uint64_t context,
       HandleSignalsState* signals_state) override;
 
  private:
   AwakableList awakable_list_ MOJO_GUARDED_BY(mutex());
 
   MOJO_DISALLOW_COPY_AND_ASSIGN(SimpleDispatcher);
 };
 
 }  // namespace system
 }  // namespace mojo
 
 #endif  // MOJO_EDK_SYSTEM_SIMPLE_DISPATCHER_H_