Apply RAW_PTR_EXCLUSION to syscall_broker::BrokerSimpleMessage pointers.

raw_ptr<> doesn't help us here since the pointers are internal, and we
may not want to invoke BRP bookkeeping in this async-safe code.

Change-Id: I19b6ccacd41d55b36661d7a99a7a122f028eb515
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4833478
Reviewed-by: Matthew Denton <mpdenton@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1191453}