Don't use exit(), use Shell::Exit() (which calls _exit() instead). This won't
run C++ static destructors, atexit() functions, etc., which can occasionally
cause flaky failures.

BUG=v8:4279
R=machenbach@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/1294913005

Cr-Commit-Position: refs/heads/master@{#30229}