[cq] Make chromium win trybot experimental

BUG=chromium:629025
NOTRY=true
NOPRESUBMIT=true
TBR=sergiyb@chromium.org

Review-Url: https://codereview.chromium.org/2165883003
Cr-Commit-Position: refs/heads/master@{#37903}

infra/config/cq.cfg

@@ -109,7 +109,10 @@ verifiers {
     }
     buckets {
       name: "master.tryserver.chromium.win"
-      builders { name: "win_chromium_compile_dbg_ng" }
+      builders {
+        name: "win_chromium_compile_dbg_ng"
+        experiment_percentage: 100
+      }
     }
   }