Convert WriteMessage...() 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 6e32d039b9e51c1ec0d54dd7636115fe1bc7e405..70f3bc32e8ac9319dc31588e274c6d3a61ad8cb8 100644
--- a/mojo/system/message_pipe_dispatcher.cc
+++ b/mojo/system/message_pipe_dispatcher.cc
@@ -179,7 +179,7 @@ MessagePipeDispatcher::CreateEquivalentDispatcherAndCloseImplNoLock() {
 }
 
 MojoResult MessagePipeDispatcher::WriteMessageImplNoLock(
-    const void* bytes,
+    UserPointer<const void> bytes,
     uint32_t num_bytes,
     std::vector<DispatcherTransport>* transports,
     MojoWriteMessageFlags flags) {
@@ -188,8 +188,6 @@ MojoResult MessagePipeDispatcher::WriteMessageImplNoLock(
 
   lock().AssertAcquired();
 
-  if (!VerifyUserPointerWithSize<1>(bytes, num_bytes))
-    return MOJO_RESULT_INVALID_ARGUMENT;
   if (num_bytes > kMaxMessageNumBytes)
     return MOJO_RESULT_RESOURCE_EXHAUSTED;