Restart crashpad_handler on Mac if it exits unexpectedly

This feature is only operative on OS X 10.10 and later, as directed by
CrashpadClient::StartHandler(). This limitation is enforced because the
handler restarter can cause the browser process to become an unreapable
pseudo-zombie on earlier OS versions if it crashes while a handler
restart is pending.

Review URL: https://codereview.chromium.org/1470073002

Cr-Commit-Position: refs/heads/master@{#361201}