This fills out Threads() in ProcessReader, gathering some information
for which there's system calls, and adds some basic tests for
ProcessReader on Fuchsia.
Bug: crashpad:196
Change-Id: I0738e77121c90a8b883267c1df0fcfc6621674d7
Reviewed-on: https://chromium-review.googlesource.com/929350
Commit-Queue: Scott Graham <scottmg@chromium.org>
Reviewed-by: Mark Mentovai <mark@chromium.org>