Cherry-pick https://codereview.chromium.org/2000403004/ to depot_tools's
bot_update.py

R=hinoka@chromium.org
BUG=613641

Review-Url: https://codereview.chromium.org/2022453003

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@300670 0039d316-1c4b-4281-b951-d872f2087c98