Add ThreadPool.abort() to stop processing early.

Make Progress more responsive in ThreadPool.join().
Change update_item() to allow decrement.

R=vadimsh@chromium.org
BUG=

Review URL: https://chromiumcodereview.appspot.com/25478010

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/swarm_client@226540 0039d316-1c4b-4281-b951-d872f2087c98
6 files changed