buildbot: Enable multiple Mac configs on RandomOrder bot.

This is temporary while we get Mac-10.12 on Intel GPU working.
During this time it will let us accurately compare 3 configurations;

 * Mac 10.9  with GPU-None  -- Currently broken
 * Mac 10.12 with GPU-None  -- Currently working
 * Mac 10.12 with GPU-Intel -- Currently broken

BUG=524758
TBR=qyearsley@chromium.org,dpranke@chromium.org,mcgreevy@chromium.org,jeffcarp@chromium.org
NOTRY=true

Review-Url: https://codereview.chromium.org/2957503002
Cr-Commit-Position: refs/heads/master@{#481768}

testing/buildbot/chromium.fyi.json

@@ -12071,6 +12071,13 @@
           "dimension_sets": [
             {
               "os": "Mac-10.9"
+            },
+            {
+              "os": "Mac-10.12"
+            },
+            {
+              "gpu": "8086:0a2e",
+              "os": "Mac-10.12"
             }
           ],
           "shards": 8