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

mojo/system/data_pipe_producer_dispatcher.cc

 // 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.
 
 #include "mojo/system/data_pipe_producer_dispatcher.h"
 
 #include "base/logging.h"
 #include "mojo/system/data_pipe.h"
 #include "mojo/system/memory.h"
 
@@ skipping 48 matching lines @@
   if (!VerifyUserPointer<uint32_t>(num_bytes))
     return MOJO_RESULT_INVALID_ARGUMENT;
   if (!VerifyUserPointerWithSize<1>(elements, *num_bytes))
     return MOJO_RESULT_INVALID_ARGUMENT;
 
   return data_pipe_->ProducerWriteData(
       elements, num_bytes, (flags & MOJO_WRITE_DATA_FLAG_ALL_OR_NONE));
 }
 
 MojoResult DataPipeProducerDispatcher::BeginWriteDataImplNoLock(
-    void** buffer,
-    uint32_t* buffer_num_bytes,
+    UserPointer<void*> buffer,
+    UserPointer<uint32_t> buffer_num_bytes,
     MojoWriteDataFlags flags) {
   lock().AssertAcquired();
 
-  if (!VerifyUserPointerWithCount<void*>(buffer, 1))
-    return MOJO_RESULT_INVALID_ARGUMENT;
-  if (!VerifyUserPointer<uint32_t>(buffer_num_bytes))
-    return MOJO_RESULT_INVALID_ARGUMENT;
-
   return data_pipe_->ProducerBeginWriteData(
       buffer, buffer_num_bytes, (flags & MOJO_WRITE_DATA_FLAG_ALL_OR_NONE));
 }
 
 MojoResult DataPipeProducerDispatcher::EndWriteDataImplNoLock(
     uint32_t num_bytes_written) {
   lock().AssertAcquired();
 
   return data_pipe_->ProducerEndWriteData(num_bytes_written);
 }
@@ skipping 11 matching lines @@
   data_pipe_->ProducerRemoveWaiter(waiter);
 }
 
 bool DataPipeProducerDispatcher::IsBusyNoLock() const {
   lock().AssertAcquired();
   return data_pipe_->ProducerIsBusy();
 }
 
 }  // namespace system
 }  // namespace mojo