buildbot: Force OS on the Mac WebKit RandomOrder bot.

The local VM is Mac-10.12 and the LayoutTests are passing, so force
swarming to also run on Mac-10.12

BUG=717344
TBR=dpranke@chromium.com,mcgreevy@chromium.org

Review-Url: https://codereview.chromium.org/2848253005
Cr-Commit-Position: refs/heads/master@{#468602}

testing/buildbot/chromium.fyi.json

@@ -12924,6 +12924,11 @@
         "name": "webkit_layout_tests",
         "swarming": {
           "can_use_on_swarming_builders": true,
+          "dimension_sets": [
+            {
+              "os": "Mac-10.12"
+            }
+          ],
           "shards": 1
         }
       }