[cr-buildbucket.cfg] Make android-marshmallow-arm64-rel try builders 16 cores

Bug: 875708
Change-Id: I057668ca809bcd9644c778690997a2ceed11ae8b
Reviewed-on: https://chromium-review.googlesource.com/1183021Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Commit-Queue: Ryan Tseng <hinoka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584679}

infra/config/global/cr-buildbucket.cfg

@@ -2357,6 +2357,8 @@ buckets {
       mixins: "goma-j300"
       name: "android-marshmallow-arm64-rel"
       dimensions: "os:Ubuntu-14.04"
+      # TODO(hinoka): Make this 16 core after crbug.com/875708
+      dimensions: "cores:"
     }
     # TODO(jbudorick): Remove this after fully migrating ANSR.
     builders {