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

mojo/public/cpp/bindings/lib/fixed_buffer.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_FIXED_BUFFER_H_
 #define MOJO_PUBLIC_CPP_BINDINGS_LIB_FIXED_BUFFER_H_
 
 #include "mojo/public/cpp/bindings/lib/buffer.h"
 #include "mojo/public/cpp/system/macros.h"
 
@@ skipping 31 matching lines @@
   // |size| should be aligned using internal::Align.
   void Initialize(void* memory, size_t size);
 
   size_t size() const { return size_; }
 
   // Grows the buffer by |num_bytes| and returns a pointer to the start of the
   // addition. The resulting address is 8-byte aligned, and the content of the
   // memory is zero-filled.
   void* Allocate(size_t num_bytes) override;
 
+  PickleBuffer* AsPickleBuffer() override;
+
  protected:
   char* ptr_;
   size_t cursor_;
   size_t size_;
 
   MOJO_DISALLOW_COPY_AND_ASSIGN(FixedBuffer);
 };
 
 class FixedBufferForTesting : public FixedBuffer {
  public:
   explicit FixedBufferForTesting(size_t size);
   ~FixedBufferForTesting() override;
 
   // Returns the internal memory owned by the Buffer to the caller. The Buffer
   // relinquishes its pointer, effectively resetting the state of the Buffer
   // and leaving the caller responsible for freeing the returned memory address
   // when no longer needed.
   void* Leak();
 
  private:
   MOJO_DISALLOW_COPY_AND_ASSIGN(FixedBufferForTesting);
 };
 
 }  // namespace internal
 }  // namespace mojo
 
 #endif  // MOJO_PUBLIC_CPP_BINDINGS_LIB_FIXED_BUFFER_H_