[CQ] Add new mac trybot to CQ experiment

NOTRY=true
TBR=sergiyb@chromium.org

Bug: chromium:822593
Change-Id: I7f3fe4f5798a4269f1900c80a4c3999122917201
Reviewed-on: https://chromium-review.googlesource.com/966141
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51978}

infra/config/cq.cfg

@@ -89,6 +89,10 @@ verifiers {
         name: "v8_mac_rel_ng_triggered"
         triggered_by: "v8_mac_rel_ng"
       }
+      builders {
+        name: "v8_mac64_rel_ng"
+        experiment_percentage: 20
+      }
       builders { name: "v8_node_linux64_rel" }
       builders { name: "v8_presubmit" }
       builders { name: "v8_win64_msvc_compile_rel" }