[CQ.cfg] 50% of chromium_presubmit builds will now run on LUCI.

R=dpranke@chromium.org, jbudorick@chromium.org

No-Try: True
Bug: 731553
Change-Id: I12cd97bfee4d1cc7bf81f5c990014a27335a1c50
Reviewed-on: https://chromium-review.googlesource.com/891666Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532539}

infra/config/cq.cfg

@@ -78,7 +78,7 @@ verifiers {
       }
       builders {
         name: "chromium_presubmit"
-        equivalent_to { bucket: "luci.chromium.try" percentage: 0 }
+        equivalent_to { bucket: "luci.chromium.try" percentage: 50 }
       }
       builders {
         name: "fuchsia_arm64"