Add win-libfuzzer-asan-rel to the CQ as a 50% experiment.

It's got the same bot pool size as the linux version of it, so jumping
from 0 to 50% should be safe.

The bot should be good to go once https://chromium-review.googlesource.com/c/chromium/tools/build/+/1287370 lands.

Bug: 893614
Change-Id: Ib6abc77bec199866dacadc0078afd50df387d0c8
Reviewed-on: https://chromium-review.googlesource.com/c/1287432Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600812}

infra/config/branch/cq.cfg

@@ -172,6 +172,10 @@ verifiers {
         path_regexp: "third_party/blink/renderer/modules/webgl/.+"
         path_regexp: "ui/gl/.+"
       }
+      builders {
+        name: "win-libfuzzer-asan-rel"
+        experiment_percentage: 50
+      }
       builders { name: "win10_chromium_x64_rel_ng" }
       builders {
         name: "win7_chromium_rel_loc_exp"