Mojo: Use new message APIs to reduce copying

mojo/public/c/system/message_pipe.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.
 
 // This file contains types/constants and functions specific to message pipes.
 //
 // Note: This header should be compilable as C.
 
 #ifndef MOJO_PUBLIC_C_SYSTEM_MESSAGE_PIPE_H_
 #define MOJO_PUBLIC_C_SYSTEM_MESSAGE_PIPE_H_
@@ skipping 290 matching lines @@
 //   |MOJO_RESULT_INVALID_ARGUMENT| if |message| was not a valid message.
 MOJO_SYSTEM_EXPORT MojoResult MojoFreeMessage(MojoMessageHandle message);
 
 // Retrieves the address of mutable message bytes for a message allocated by
 // either |MojoAllocMessage()| or |MojoReadMessageNew()|.
 //
 // Returns:
 //   |MOJO_RESULT_OK| if |message| is a valid message object. |*buffer| will
 //       be updated to point to mutable message bytes.
 //   |MOJO_RESULT_INVALID_ARGUMENT| if |message| is not a valid message object.
+//
+// NOTE: A returned buffer address is always guaranteed to be 8-byte aligned.
 MOJO_SYSTEM_EXPORT MojoResult MojoGetMessageBuffer(MojoMessageHandle message,
                                                    void** buffer);  // Out
 
 #ifdef __cplusplus
 }  // extern "C"
 #endif
 
 #endif  // MOJO_PUBLIC_C_SYSTEM_MESSAGE_PIPE_H_