This patch also moves Shell::Exit to base::OS::ExitProcess.

Bug: chromium:824214
Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: I6565eebe9332557bbfb8e67c88890b7099b1db98
Reviewed-on: https://chromium-review.googlesource.com/975403Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52150}