Make it possible to remove an Awakable with a specific "context" from a Dispatcher.

mojo/edk/system/local_message_pipe_endpoint.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_LOCAL_MESSAGE_PIPE_ENDPOINT_H_
 #define MOJO_EDK_SYSTEM_LOCAL_MESSAGE_PIPE_ENDPOINT_H_
 
 #include "mojo/edk/system/awakable_list.h"
 #include "mojo/edk/system/handle_signals_state.h"
 #include "mojo/edk/system/message_in_transit_queue.h"
@@ skipping 26 matching lines @@
                          uint32_t* num_handles,
                          MojoReadMessageFlags flags) override;
   HandleSignalsState GetHandleSignalsState() const override;
   MojoResult AddAwakable(Awakable* awakable,
                          MojoHandleSignals signals,
                          bool force,
                          uint64_t context,
                          HandleSignalsState* signals_state) override;
   void RemoveAwakable(Awakable* awakable,
                       HandleSignalsState* signals_state) override;
+  void RemoveAwakableWithContext(Awakable* awakable,
+                                 uint64_t context,
+                                 HandleSignalsState* signals_state) override;
 
   // This is only to be used by |MessagePipe|:
   MessageInTransitQueue* message_queue() { return &message_queue_; }
 
  private:
   bool is_open_;
   bool is_peer_open_;
 
   // Queue of incoming messages.
   MessageInTransitQueue message_queue_;
   AwakableList awakable_list_;
 
   MOJO_DISALLOW_COPY_AND_ASSIGN(LocalMessagePipeEndpoint);
 };
 
 }  // namespace system
 }  // namespace mojo
 
 #endif  // MOJO_EDK_SYSTEM_LOCAL_MESSAGE_PIPE_ENDPOINT_H_