Enable graceful shutdown of slaves.
Enable graceful shutdown of slaves.

With this change, touching the file build/slave/shutdown.stamp
will gracefully take a slave offline.

BUG=
R=iannucci@chromium.org

Review URL: https://codereview.chromium.org/217043004

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@260342 0039d316-1c4b-4281-b951-d872f2087c98

eunchong/build,eunchong/build,eunchong/build,eunchong/build