20% equivalence for win7_chromium_rel_ng

Ran it at 20% experiment for a while, so it should be fine to do this
capacity wise.

NOTRY=true

Bug: 756689
Change-Id: Ia96c0e383f0c5d631759733faf2ab8213f7796f6
Reviewed-on: https://chromium-review.googlesource.com/1080269
Commit-Queue: Stephen Martinis <martiniss@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#563101}

infra/config/branch/cq.cfg

@@ -96,7 +96,7 @@ verifiers {
       name: "master.tryserver.chromium.win"
       builders {
         name: "win7_chromium_rel_ng"
-        equivalent_to { bucket: "luci.chromium.try" percentage: 0 }
+        equivalent_to { bucket: "luci.chromium.try" percentage: 20 }
       }
       builders {
         name: "win7_chromium_rel_loc_exp"