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

mojo/system/data_pipe_producer_dispatcher.cc

Index: mojo/system/data_pipe_producer_dispatcher.cc
diff --git a/mojo/system/data_pipe_producer_dispatcher.cc b/mojo/system/data_pipe_producer_dispatcher.cc
index ec99549b5438d4916e0ddc87a983dbf3db7b2699..b54f2d28bc41cd6a33106a4fa244eb9d1bb02069 100644
--- a/mojo/system/data_pipe_producer_dispatcher.cc
+++ b/mojo/system/data_pipe_producer_dispatcher.cc
@@ -66,16 +66,11 @@ MojoResult DataPipeProducerDispatcher::WriteDataImplNoLock(
 }
 
 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));
 }