auto_reboot: mark buildbot as being shutdown
auto_reboot: mark buildbot as being shutdown

AutoRebootBuildSlave.buildFinished method starts a shutdown process
without setting graceful shutdown flag (SlaveStatus.setGraceful). As a result, a
build slave might accept a new job (AbstractBuildSlave.canStartBuild)
right before being rebooted. This change attempts to fix it.

R=stip@chromium.org, agable@chromium.org
BUG=421566

Review URL: https://codereview.chromium.org/638843003

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@292405 0039d316-1c4b-4281-b951-d872f2087c98

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