Some ChromeOS builders are unsafe to run multiple times in parallel, any build that commits uprevs or uploads prebuilts in particular.

buildbot was configured to enforce single build at a time for these configs, but swarming isn't capable of doing that. The LUCI scheduler will only schedule a single build at a time, but they can also be scheduled through alternative means.

One way to enforce single build at a time is to have adjust CancelObsoleteSlaveBuilds (which already cancels old build slaves) to also cancel previous builds which are still in progress. We would, of course, need a new build config value to turn this behavior on or off.