Final removal of IPC macros in //remoting

A little over a year ago we were asked to remove our IPC macro usages
as part of a broad clean-up effort and to modernize our IPC impl by
moving to Mojo.  Fast-forward ~1 year and 20 CLs later and the end is
here (for IPC macros that is).

This CL replaces the last remaining IPC message in //remoting with a
shiny new Mojo interface. This message was used by the
WorkerProcessLauncher which would use it to ask a child process to
crash (exit) and produce a dump. I've updated the callsites to use
the new interface, tested the impl to make sure it will crash the
child process when requested, fixed the tests, and removed all
references to our old IPC macro file.

This should be it, no more crufty IPC in CRD!
<skeletor_smashing_mirror_as_they_exit.gif>

Bug: b:178114059
Change-Id: Ifbba870fbdb58c7731afd04559d3943e1d89eae0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3799549
Reviewed-by: Yuwei Huang <yuweih@chromium.org>
Commit-Queue: Joe Downing <joedow@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1035779}