Revert "cq: drop linux-goma-rbe-staging-rel experiment."

This reverts commit a56c7856.

Reason for revert: 
added more capacity: https://chrome-internal-review.googlesource.com/c/infradata/config/+/666063
https://bugs.chromium.org/p/chromium/issues/detail?id=855319#c35

Original change's description:
> cq: drop linux-goma-rbe-staging-rel experiment.
> 
> TBR=martiniss@chromium.org
> NOTRY=true
> 
> Bug: 855319
> Change-Id: Ia760d2ff0e5c7bfe6cf1508464d0c1e5bc1e2026
> Reviewed-on: https://chromium-review.googlesource.com/1194325
> Reviewed-by: John Budorick <jbudorick@chromium.org>
> Commit-Queue: John Budorick <jbudorick@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#586731}

TBR=martiniss@chromium.org,jbudorick@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 855319
Change-Id: Ibd6d62bf793809162ba27a113d5c9429808702e7
Reviewed-on: https://chromium-review.googlesource.com/1220058Reviewed-by: Fumitoshi Ukai <ukai@chromium.org>
Commit-Queue: Fumitoshi Ukai <ukai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590579}

infra/config/branch/cq.cfg

@@ -70,6 +70,11 @@ verifiers {
         name: "linux-dcheck-off-rel"
         experiment_percentage: 10
       }
+      # https://crbug.com/855319
+      builders {
+        name: "linux-goma-rbe-staging-rel"
+        experiment_percentage: 10
+      }
       builders { name: "linux-jumbo-rel" }
       # TODO(crbug.com/646404): Scale this up to 100% mandatory.
       builders {