infra: register "GPU Mac Builder" and its testers in buildbucket.

Testers:
 * "Mac Release (Intel)"
 * "Mac Retina Release (AMD)"

R=kbr@chromium.org

Bug: 790063,790077,790066
Change-Id: Iebe7718ee74428f7d1f89d590ca59c8b1dacd524
No-Try: True
Reviewed-on: https://chromium-review.googlesource.com/994114Reviewed-by: Ryan Tseng <hinoka@chromium.org>
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547907}

infra/config/global/cr-buildbucket.cfg

@@ -282,6 +282,14 @@ builder_mixins {
   }
 }
 
+builder_mixins {
+  name: "mac-gpu-ci"
+  mixins: "mac"
+  recipe {
+    properties: "mastername:chromium.gpu"
+  }
+}
+
 builder_mixins {
   name: "mac-gpu-fyi-ci"
   mixins: "mac"
@@ -695,6 +703,23 @@ buckets {
       mixins: "mac-ci"
     }
 
+    builders {
+      name: "GPU Mac Builder"
+      mixins: "mac-gpu-ci"
+    }
+    builders {
+      name: "Mac Release (Intel)"
+      mixins: "mac-gpu-ci"
+      # TODO(tandrii,kbr): this builder doesn't actually run tests locally,
+      # and so can perhaps run on cheaper linux hardware.
+    }
+    builders {
+      name: "Mac Retina Release (AMD)"
+      mixins: "mac-gpu-ci"
+      # TODO(tandrii,kbr): this builder doesn't actually run tests locally,
+      # and so can perhaps run on cheaper linux hardware.
+    }
+
     # chromium.gpu.fyi
     builders {
       name: "GPU FYI Mac Builder"