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

mojo/public/cpp/bindings/lib/buffer.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_PUBLIC_CPP_BINDINGS_LIB_BUFFER_H_
 #define MOJO_PUBLIC_CPP_BINDINGS_LIB_BUFFER_H_
 
 #include <stddef.h>
 
 namespace mojo {
 namespace internal {
 
+class PickleBuffer;
+
 // Buffer provides a way to allocate memory. Allocations are 8-byte aligned and
 // zero-initialized. Allocations remain valid for the lifetime of the Buffer.
 class Buffer {
  public:
   virtual ~Buffer() {}
   virtual void* Allocate(size_t num_bytes) = 0;
+
+  // TODO(rockot): Remove this. It's a hack to get a PickleBuffer in
+  // Serialize_ calls without having to update every call site.
+  virtual PickleBuffer* AsPickleBuffer() = 0;
 };
 
 }  // namespace internal
 }  // namespace mojo
 
 #endif  // MOJO_PUBLIC_CPP_BINDINGS_LIB_BUFFER_H_