Auto-reboot slaves on the trybot waterfall.
Auto-reboot slaves on the trybot waterfall.

This helps work around kernel bugs in GCE where processes are
unkillable. It also perhaps makes sense in general because we don't
trust the code that's running on the tryserver (someone could be
running a bad chromite patch on there and we wouldn't want the state to
persist.)

BUG=chromium:444732
TEST=make restart

Review URL: https://codereview.chromium.org/824813002

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@293496 0039d316-1c4b-4281-b951-d872f2087c98

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