Extends MultiprocessExec to support running functions registered via
CRASHPAD_CHILD_TEST_MAIN() as the main of a new child process.
Additionally, implements Fuchsia exit code checking, and adds a
CRASHPAD_CHILD_TEST_MAIN()-based test for that.
Bug: crashpad:196, crashpad:215
Change-Id: I49ce3f4d95a3b9823813e6df5a602cee2583bcf8
Reviewed-on: https://chromium-review.googlesource.com/879563
Commit-Queue: Scott Graham <scottmg@chromium.org>
Reviewed-by: Mark Mentovai <mark@chromium.org>
