[mojo] Use base::Pickle for Message storage.

mojo/public/cpp/bindings/lib/message_internal.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_PUBLIC_CPP_BINDINGS_LIB_MESSAGE_INTERNAL_H_
 #define MOJO_PUBLIC_CPP_BINDINGS_LIB_MESSAGE_INTERNAL_H_
 
 #include "mojo/public/cpp/bindings/lib/bindings_internal.h"
 
 namespace mojo {
@@ skipping 17 matching lines @@
 static_assert(sizeof(MessageHeader) == 24, "Bad sizeof(MessageHeader)");
 
 struct MessageHeaderWithRequestID : MessageHeader {
   // Only used if either kMessageExpectsResponse or kMessageIsResponse is set in
   // order to match responses with corresponding requests.
   uint64_t request_id;
 };
 static_assert(sizeof(MessageHeaderWithRequestID) == 32,
               "Bad sizeof(MessageHeaderWithRequestID)");
 
-struct MessageData {
-  MessageHeader header;
-};
-
-static_assert(sizeof(MessageData) == sizeof(MessageHeader),
-              "Bad sizeof(MessageData)");
-
 #pragma pack(pop)
 
 }  // namespace internal
 }  // namespace mojo
 
 #endif  // MOJO_PUBLIC_CPP_BINDINGS_LIB_MESSAGE_INTERNAL_H_