GitLab

This adds copy of
https://luci-milo-dev.appspot.com/p/chromium/builders/ci/linux-rel-swarming
but use chromium-swarm-staging.appspot.com.

I don't have confidence whether this builder runs correctly at first,
but let me try to see what happens.

Bug: b/144318226
Change-Id: I3fba44f82619e6303fa2685063658eafbcbdf3b7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1989300
Auto-Submit: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: Yoshisato Yanagisawa <yyanagisawa@google.com>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#745826}