GitLab

Since it's used more generally for TaskSource. Coming CL will make use of
TaskSourceSortKey to schedule jobs more fairly.

Change-Id: I2084f56bf7082e39ed16a4afb19a9356f0390295
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2377995Reviewed-by: François Doray <fdoray@chromium.org>
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#803730}