Convert BeginWriteData...() to use the new user pointer handling (see r285350).

mojo/system/data_pipe_producer_dispatcher.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_SYSTEM_DATA_PIPE_PRODUCER_DISPATCHER_H_
 #define MOJO_SYSTEM_DATA_PIPE_PRODUCER_DISPATCHER_H_
 
 #include "base/basictypes.h"
 #include "base/compiler_specific.h"
 #include "base/memory/ref_counted.h"
@@ skipping 23 matching lines @@
 
   // |Dispatcher| protected methods:
   virtual void CancelAllWaitersNoLock() OVERRIDE;
   virtual void CloseImplNoLock() OVERRIDE;
   virtual scoped_refptr<Dispatcher>
       CreateEquivalentDispatcherAndCloseImplNoLock() OVERRIDE;
   virtual MojoResult WriteDataImplNoLock(const void* elements,
                                          uint32_t* num_bytes,
                                          MojoWriteDataFlags flags) OVERRIDE;
   virtual MojoResult BeginWriteDataImplNoLock(
-      void** buffer,
-      uint32_t* buffer_num_bytes,
+      UserPointer<void*> buffer,
+      UserPointer<uint32_t> buffer_num_bytes,
       MojoWriteDataFlags flags) OVERRIDE;
   virtual MojoResult EndWriteDataImplNoLock(
       uint32_t num_bytes_written) OVERRIDE;
   virtual MojoResult AddWaiterImplNoLock(Waiter* waiter,
                                          MojoHandleSignals signals,
                                          uint32_t context) OVERRIDE;
   virtual void RemoveWaiterImplNoLock(Waiter* waiter) OVERRIDE;
   virtual bool IsBusyNoLock() const OVERRIDE;
 
   // Protected by |lock()|:
   scoped_refptr<DataPipe> data_pipe_;  // This will be null if closed.
 
   DISALLOW_COPY_AND_ASSIGN(DataPipeProducerDispatcher);
 };
 
 }  // namespace system
 }  // namespace mojo
 
 #endif  // MOJO_SYSTEM_DATA_PIPE_PRODUCER_DISPATCHER_H_