[LUCI] increase linux_chromium_headless_rel percentage to 10%

TBR=jbudorick@chromium.org

Bug: 731559
Change-Id: I6ed4bfa7b6c22f41a9fd65732cabc22064eb8162
Reviewed-on: https://chromium-review.googlesource.com/1073620Reviewed-by: Shenghua Zhang <shenghuazhang@chromium.org>
Commit-Queue: Shenghua Zhang <shenghuazhang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561948}

infra/config/branch/cq.cfg

@@ -106,7 +106,7 @@ verifiers {
       name: "master.tryserver.chromium.linux"
       builders {
         name: "linux_chromium_headless_rel"
-        equivalent_to { bucket: "luci.chromium.try" percentage: 0 }
+        equivalent_to { bucket: "luci.chromium.try" percentage: 10 }
       }
     }
     buckets {