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

mojo/system/message_pipe.cc

Index: mojo/system/message_pipe.cc
diff --git a/mojo/system/message_pipe.cc b/mojo/system/message_pipe.cc
index a301c8f8fd16fe8457ee7542deef27d8c750cd4f..5385a2b2500ed1fa0228ac2c4c6a4a2d308cada2 100644
--- a/mojo/system/message_pipe.cc
+++ b/mojo/system/message_pipe.cc
@@ -67,7 +67,7 @@ void MessagePipe::Close(unsigned port) {
 // TODO(vtl): Handle flags.
 MojoResult MessagePipe::WriteMessage(
     unsigned port,
-    const void* bytes,
+    UserPointer<const void> bytes,
     uint32_t num_bytes,
     std::vector<DispatcherTransport>* transports,
     MojoWriteMessageFlags flags) {
@@ -76,9 +76,7 @@ MojoResult MessagePipe::WriteMessage(
       GetPeerPort(port),
       make_scoped_ptr(new MessageInTransit(
           MessageInTransit::kTypeMessagePipeEndpoint,
-          MessageInTransit::kSubtypeMessagePipeEndpointData,
-          num_bytes,
-          bytes)),
+          MessageInTransit::kSubtypeMessagePipeEndpointData, num_bytes, bytes)),
       transports);
 }