veyron-browser: Ensuring tail commands run by shell::wait_for are killed properly when script exits.
veyron-browser: Ensuring tail commands run by shell::wait_for are killed
properly when script exits.

Fixes issues with presubmitter where hanging tail procs caused the presubmit
to hang.

Change-Id: I5153fdacd7ecdda4da744723a146f7ff7cccfe4e
vanadium/browser,vanadium/browser,vanadium/browser,vanadium/browser