Reland "[cr-buildbucket] Clear 'cores' dimension for ios-* builders."

This is a reland of be5a2a38

TBR=vadimsh@chromium.org, nodir@chromium.org, sergeyberezin@chromium.org

Original change's description:
> [cr-buildbucket] Clear 'cores' dimension for ios-* builders.
>
> ios-* builders may run on a mix of VMs with cores:8 and Mac Mini's with cores:4.
>
> Bug: 731546
> Change-Id: I056fbb5bd80ca50ef8005aa8bb99643853acd923
> Reviewed-on: https://chromium-review.googlesource.com/1024729
> Reviewed-by: Vadim Shtayura <vadimsh@chromium.org>
> Reviewed-by: Nodir Turakulov <nodir@chromium.org>
> Commit-Queue: Sergey Berezin <sergeyberezin@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#553160}

Bug: 731546
Change-Id: I97848d9fccbc3c4d527d9a205150c7f6ffd920dd
Reviewed-on: https://chromium-review.googlesource.com/1025183Reviewed-by: Shuotao Gao <stgao@chromium.org>
Commit-Queue: Shuotao Gao <stgao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553229}

infra/config/global/cr-buildbucket.cfg

@@ -286,6 +286,8 @@ builder_mixins {
 builder_mixins {
   name: "ios-ci"
   mixins: "mac-ci"
+  # ios bots may run both on Mac Minis with cores:4 and VMs with cores:8.
+  dimensions: "cores:"
   recipe {
     name: "ios/unified_builder_tester"
   }
@@ -294,6 +296,8 @@ builder_mixins {
 builder_mixins {
   name: "ios-try"
   mixins: "mac-try"
+  # ios bots may run both on Mac Minis with cores:4 and VMs with cores:8.
+  dimensions: "cores:"
   recipe {
     name: "ios/try"
   }