ipc: Add the inner class OutputElement to Channel.

ipc/ipc_channel.h

 // Copyright (c) 2012 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 IPC_IPC_CHANNEL_H_
 #define IPC_IPC_CHANNEL_H_
 
 #include <string>
 
 #if defined(OS_POSIX)
@@ skipping 222 matching lines @@
   static void SetGlobalPid(int pid);
 #endif
 
 #if defined(OS_ANDROID)
   // Most tests are single process and work the same on all platforms. However
   // in some cases we want to test multi-process, and Android differs in that it
   // can't 'exec' after forking. This callback resets any data in the forked
   // process such that it acts similar to if it was exec'd, for tests.
   static void NotifyProcessForkedForTesting();
 #endif
+
+ protected:
+  // An OutputElement is a wrapper around a Message or raw buffer while it is
+  // waiting to be passed to the system's underlying IPC mechanism.
+  class OutputElement {
+   public:
+    // Takes ownership of message.
+    OutputElement(Message* message);
+    // Takes ownership of the buffer. |buffer| is freed via free(), so it
+    // must be malloced.
+    OutputElement(void* buffer, size_t length);
+    ~OutputElement();
+    size_t size() const { return message_ ? message_->size() : length_; }
+    const void* data() const { return message_ ? message_->data() : buffer_; }
+    const Message* get_message() const { return message_.get(); }
+
+   private:
+    scoped_ptr<const Message> message_;
+    void* buffer_;
+    size_t length_;
+  };
 };
 
 #if defined(OS_POSIX)
 // SocketPair() creates a pair of socket FDs suitable for using with
 // IPC::Channel.
 IPC_EXPORT bool SocketPair(int* fd1, int* fd2);
 #endif
 
 }  // namespace IPC
 
 #endif  // IPC_IPC_CHANNEL_H_