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

mojo/system/message_pipe_dispatcher.cc

Index: mojo/system/message_pipe_dispatcher.cc
diff --git a/mojo/system/message_pipe_dispatcher.cc b/mojo/system/message_pipe_dispatcher.cc
index 484613ca5d195636feadd64c17b7057047ca2706..6e32d039b9e51c1ec0d54dd7636115fe1bc7e405 100644
--- a/mojo/system/message_pipe_dispatcher.cc
+++ b/mojo/system/message_pipe_dispatcher.cc
@@ -198,20 +198,12 @@ MojoResult MessagePipeDispatcher::WriteMessageImplNoLock(
 }
 
 MojoResult MessagePipeDispatcher::ReadMessageImplNoLock(
-    void* bytes,
-    uint32_t* num_bytes,
+    UserPointer<void> bytes,
+    UserPointer<uint32_t> num_bytes,
     DispatcherVector* dispatchers,
     uint32_t* num_dispatchers,
     MojoReadMessageFlags flags) {
   lock().AssertAcquired();
-
-  if (num_bytes) {
-    if (!VerifyUserPointer<uint32_t>(num_bytes))
-      return MOJO_RESULT_INVALID_ARGUMENT;
-    if (!VerifyUserPointerWithSize<1>(bytes, *num_bytes))
-      return MOJO_RESULT_INVALID_ARGUMENT;
-  }
-
   return message_pipe_->ReadMessage(port_, bytes, num_bytes, dispatchers,
                                     num_dispatchers, flags);
 }