[infra] Request cores:16 on android-marshmallow-arm64-rel

To ensure it gets the right number of cores when migrated to
builderless. A no-op now, since builder:android-marshmallow-arm64-rel
currently implies cores:16.

Bug: 956776
Change-Id: I785938a91a7cf493ece0b0579cc9d8f106f673ee
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1606637
Auto-Submit: smut <smut@google.com>
Reviewed-by: Ben Pastene <bpastene@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659061}

infra/config/cr-buildbucket.cfg

@@ -3510,8 +3510,7 @@ 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:"
+      dimensions: "cores:16"
     }
     builders {
       mixins: "android-try"
@@ -4034,6 +4033,7 @@ buckets {
     builders {
       mixins: "win-try"
       mixins: "goma-j300"
+      # TODO(crbug/881609): Add 'dimensions: "ssd:1"'.
       name: "win7-rel"
       execution_timeout_secs: 16200  # 4.5h
     }