Add LUCI "WebKit Mac10.11 (dbg)" builder to buildbucket.

R=jchinlee@chromium.org

No-Try: True
Bug: 790325
Change-Id: I5f3b17a7a25beaadb4c8b4864ddb46ef9500f67b
Reviewed-on: https://chromium-review.googlesource.com/945330
Reviewed-by: Jao-ke Chin-Lee <jchinlee@chromium.org>
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#540432}

infra/config/global/cr-buildbucket.cfg

@@ -313,6 +313,13 @@ buckets {
       dimensions: "cpu:x86-64"
       dimensions: "cores:8"
     }
+    builders {
+      name: "WebKit Mac10.11 (dbg)"
+      mixins: "webkit-ci"
+      dimensions: "os:Mac-10.11"
+      dimensions: "cpu:x86-64"
+      dimensions: "cores:8"
+    }
 
     # Win bots.
     builders {