[cq] Temporarily make failing chromium trybot experimental

BUG=chromium:648153
NOTRY=true
NOPRESUBMIT=true
TBR=emso@chromium.org

Review-Url: https://codereview.chromium.org/2352573002
Cr-Commit-Position: refs/heads/master@{#39494}

infra/config/cq.cfg

@@ -57,7 +57,10 @@ verifiers {
         name: "v8_linux_arm_rel_ng_triggered"
         triggered_by: "v8_linux_arm_rel_ng"
       }
-      builders { name: "v8_linux_chromium_gn_rel" }
+      builders {
+        name: "v8_linux_chromium_gn_rel"
+        experiment_percentage: 100
+      }
       builders { name: "v8_linux_dbg_ng" }
       builders {
         name: "v8_linux_dbg_ng_triggered"