Use a default priority of 30 to match CQ jobs and not starve.
The user can also carefully add a higher priotity for small tasks.

Bug: angleproject:5177
Change-Id: I8e8460f096102423429b19e2107673f58d73805a
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2475454
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>