Change the server_process stop timeout to be 0.0 by default.

The only time we want to gracefully shutdown is in the stop_when_done
codepath in driver.run_test. Otherwise, we don't read the output anyways,
so there's no point in taking the time to cleanly shutdown.

This fixes the issue where some of us were frequently seeing
"stopping content_shell(pid XXXX) timed out, killing it" because
content_shell was taking longer than 3 seconds to shutdown.

Review URL: https://codereview.chromium.org/26633002

git-svn-id: svn://svn.chromium.org/blink/trunk@159217 bbb929c8-8fbe-4397-9dbb-9b2b20218538