GitLab

This relands parts of https://codereview.chromium.org/1947183004/
Now that clobber runs as a separate step before runhooks, swarming should work
on gn clobber bots.

BUG=498033

Review-Url: https://codereview.chromium.org/2043743003
Cr-Commit-Position: refs/heads/master@{#398148}