Create UntrustedStub

The behavior of rejecting an incoming NamedMojoIpcServer connection is
platform specific and difficult to test. On Linux, dropping the platform
handle associated with the endpoint is observed by the remote, allowing
disconnect handlers to fire. On MacOS, however, it does not seem
possible to detect this.

This difference makes testing the rejection of untrusted callers
difficult; the related AppShutdown tests have been failing on Mac since
we stopped relying on a timeout-based assertions.

Bug: 342180612
Change-Id: Ie2b35c9d19379c59b9434790d489af07731e1ee8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5758607
Reviewed-by: Joshua Pawlicki <waffles@chromium.org>
Commit-Queue: Noah Rose Ledesma <noahrose@google.com>
Cr-Commit-Position: refs/heads/main@{#1336611}