Run win_chromium_x64_rel_ng at 2% experiment for migration data.

BUG=731502

Change-Id: I12565be7b445c34d2ac16a3b1fca7feb9c88a2b4
Reviewed-on: https://chromium-review.googlesource.com/1014403Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Jao-ke Chin-Lee <jchinlee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551132}

infra/config/branch/cq.cfg

@@ -153,6 +153,12 @@ verifiers {
         name: "win10_chromium_x64_rel_ng"
         equivalent_to { bucket: "luci.chromium.try" percentage: 0 }
       }
+      builders {
+        # Temporary addition to gather data for LUCI migration.
+        # https://crbug.com/731502
+        name: "win_chromium_x64_rel_ng"
+        experiment_percentage: 2
+      }
       builders {
         name: "win7_chromium_rel_loc_exp"
         experiment_percentage: 20