Bump luci percentage of android_arm64_dbg_recipe to 50%.

Its pool's got 30 bots. Should hopefully be enough for 50% of a
compile-only bot.

Bug: 731962
Change-Id: I7d27a8edf66170442ec6fcc66575e83ee355f9c4
Reviewed-on: https://chromium-review.googlesource.com/993154Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Benjamin Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547764}

infra/config/branch/cq.cfg

@@ -33,7 +33,7 @@ verifiers {
       name: "master.tryserver.chromium.android"
       builders {
         name: "android_arm64_dbg_recipe"
-        equivalent_to { bucket: "luci.chromium.try" percentage: 10 }
+        equivalent_to { bucket: "luci.chromium.try" percentage: 50 }
       }
       builders {
         name: "android_clang_dbg_recipe"