Mojo: Add a MessageInTransit::Swap() method and remove the "PassContents" ctor.

mojo/system/message_in_transit_queue.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_SYSTEM_MESSAGE_QUEUE_H_
 #define MOJO_SYSTEM_MESSAGE_QUEUE_H_
 
 #include <deque>
 
 #include "base/macros.h"
 #include "base/memory/scoped_ptr.h"
 #include "mojo/system/message_in_transit.h"
 #include "mojo/system/system_impl_export.h"
 
 namespace mojo {
 namespace system {
 
 // A simple queue for |MessageInTransit|s (that owns its messages).
 // This class is not thread-safe.
 // TODO(vtl): Write tests.
 class MOJO_SYSTEM_IMPL_EXPORT MessageInTransitQueue {
  public:
   MessageInTransitQueue();
-
-  struct PassContents {};
-  // Constructor that takes over the contents of another
-  // |MessageInTransitQueue|, leaving it empty.
-  MessageInTransitQueue(PassContents, MessageInTransitQueue* other);
-
   ~MessageInTransitQueue();
 
   bool IsEmpty() const { return queue_.empty(); }
 
   void AddMessage(scoped_ptr<MessageInTransit> message) {
     queue_.push_back(message.release());
   }
 
   scoped_ptr<MessageInTransit> GetMessage() {
     MessageInTransit* rv = queue_.front();
     queue_.pop_front();
     return make_scoped_ptr(rv);
   }
 
   MessageInTransit* PeekMessage() { return queue_.front(); }
 
   void DiscardMessage() {
     delete queue_.front();
     queue_.pop_front();
   }
 
   void Clear();
 
+  // Efficiently swaps contents with |*other|.
+  void Swap(MessageInTransitQueue* other);
+
  private:
   // TODO(vtl): When C++11 is available, switch this to a deque of
   // |scoped_ptr|/|unique_ptr|s.
   std::deque<MessageInTransit*> queue_;
 
   DISALLOW_COPY_AND_ASSIGN(MessageInTransitQueue);
 };
 
 }  // namespace system
 }  // namespace mojo
 
 #endif  // MOJO_SYSTEM_MESSAGE_QUEUE_H_