CROS: Tryserver pools multiple 'pre-cq' configs.
Previously, there has been exactly one 'pre-cq-group' launcher. This is given
special pooling, display, and ordering treatment.
https://chromium-review.googlesource.com/#/c/227890 introduces multiple 'pre-cq'
builders. This CL updates the tryserver logic to provide all of these builders
with the same pooling and display treatment.
BUG=chromium:384169
TEST=local
- Started locally, verified waterfall ordering, slave allocation, and web
buildslave grouping.