re roll process:

the recipe revision is specified in cr-buildbucket.cfg
https://cs.chromium.org/chromium/infra/appengine/cr-buildbucket/proto/project_config.proto?q=file:buildbucket.%2Bproject&sq=package:chromium&l=71
and defaults to the latest.

but there is no roller that knows how to update cr-buildbucket.cfg files

 Issue 786868  has been merged into this issue.

I didn't see this dup, since there is no "build revision" in LUCI.

I guess what you mean here is that recipe may be located in other repos than build. I've found the corresponding revision in the log after line ending with "$ git rev-parse HEAD", e.g. for https://chromium-swarm.appspot.com/task?id=39bf9736b0e2f510&refresh=10&show_raw=1&wide_logs=true, it's 093f053280a2b672e7096ce1ee9135bbd41926c7.

What I was actually looking, when filing this bug, was revision of depot_tools recipe dep, which can be found by looking for "INFO:recipe_engine.fetch:Checking out", which is followed by a revision and the name of the corresponding repository. Since there can be multiple deps, one has to carefully select the right one.