Send 25% of CQ tryjobs to LUCI's mac_chromium_compile_dbg_ng.

Also, stop experimenting with mac_chromium_dbg_ng.

R=jchinlee

Bug: 731545, 731476
No-Try: True
Change-Id: I89d3b7eb95f4ec76909101e802f70d2627cf4602
Reviewed-on: https://chromium-review.googlesource.com/947469Reviewed-by: Jao-ke Chin-Lee <jchinlee@chromium.org>
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#540702}

infra/config/branch/cq.cfg

@@ -141,22 +141,12 @@ verifiers {
       }
       builders {
         name: "mac_chromium_compile_dbg_ng"
-        equivalent_to { bucket: "luci.chromium.try" percentage: 0 }
+        equivalent_to { bucket: "luci.chromium.try" percentage: 25 }
       }
       builders {
         name: "mac_chromium_rel_ng"
         equivalent_to { bucket: "luci.chromium.try" percentage: 0 }
       }
-      builders {
-        # The only purpose of this builder here is get data for migrating this
-        # builder to LUCI, see https://crbug.com/731476.
-        # 1% of ~90 cq runs/peak hour => builds per peak hour, which currently
-        # is fine given shared 5 VMs with another optional CQ builder.
-        # TODO(tandrii): remove this builder once enough data has been
-        # collected.
-        name: "mac_chromium_dbg_ng"
-        experiment_percentage: 3
-      }
     }
     buckets {
       name: "master.tryserver.chromium.win"