qyearsley@, who'd be a good owner for this?

(As author of try_flag.py I am happy to roll a patch but qyearsley or robertma can probably advise on what changes are needed.)

I think this is caused by LUCI migration.

Linux and Mac bots are probably fully migrated and have to be triggered from the LUCI bucket and the old Buildbot buckets are no longer available.

i.e. change these lines:
https://cs.chromium.org/chromium/src/third_party/WebKit/Tools/Scripts/webkitpy/layout_tests/try_flag.py?q=try_flag.py&sq=package:chromium&l=30
to luci.chromium.try

However, win7 not running layout tests is a separate  issue 831585  (layout tests are currently disabled on win7 CQ bots due to infra issues). A potential workaround is to use win7_blink_rel instead.

This might be a good timing to switch to builders.json, which is a more proper fix, depending on how urgent the problem is (skobes@?).

Thanks for the pointers!  I'll look into using builders.json.  I think this is not urgent.

Previously builders.json didn't have a master/bucket field, so it was a bit ugly to add non-tryserver.blink bots to the file. But I have recently added "bucket" as an optional field, so you can have CQ bots there as well and at least use that file for builder->bucket mapping.