If this logic is going to be removed from swarming.py then I think it should be replaced by logic which lives in chromium/src and is invoked by the new trigger script mechanism that martiniss@ just added. It's still not feasible to run Chromium trybots against changes to recipes, and this is the sort of deep change that absolutely needs to be tested before deployment.

I would suggest adding a default trigger script into chromium/src which is used by the recipes if the user doesn't specify one, and which handles sharding.

Blocking this on the resolution of  Issue 781057 , which is adding the first Chromium-side trigger script which will be invoked all the time on some of the CQ bots.