[CQ] Make gn bot temporarily experimental.

TBR=sergiyb@chromium.org, dpranke@chromium.org
BUG=chromium:481693
LOG=n
NOTRY=true
NOPRESUBMIT=true

Review URL: https://codereview.chromium.org/1829673002

Cr-Commit-Position: refs/heads/master@{#35026}

infra/config/cq.cfg

@@ -36,7 +36,10 @@ verifiers {
       }
       builders { name: "v8_linux_arm64_rel" }
       builders { name: "v8_linux_arm_rel" }
-      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"