Swarming protos

Source:

• Repo: https://chromium.googlesource.com/infra/luci/luci-py
• Path: appengine/swarming/proto
• Revision: 6b3d8a55917cef981fac9912bc63e25569dff507

Updating

Run ./update.sh. It will pull in protos, regenerate go code and update the revision in this file.