buildbot: Force OS on the Mac WebKit RandomOrder bot.

testing/buildbot/chromium.fyi.json

 {
   "Android Asan Builder Tests (dbg)": {
     "gtest_tests": [
       {
         "args": [
           "--tool=asan"
         ],
         "swarming": {
           "can_use_on_swarming_builders": false
         },
@@ skipping 12906 matching lines @@
       }
     ]
   },
   "WebKit Mac - RandomOrder": {
     "isolated_scripts": [
       {
         "isolate_name": "webkit_layout_tests_exparchive",
         "name": "webkit_layout_tests",
         "swarming": {
           "can_use_on_swarming_builders": true,
+          "dimension_sets": [
+            {
+              "os": "Mac-10.12"
+            }
+          ],
           "shards": 1
         }
       }
     ]
   },
   "WebKit Win - RandomOrder": {
     "isolated_scripts": [
       {
         "isolate_name": "webkit_layout_tests_exparchive",
         "name": "webkit_layout_tests",
@@ skipping 237 matching lines @@
       },
       {
         "swarming": {
           "can_use_on_swarming_builders": false
         },
         "test": "views_unittests"
       }
     ]
   }
 }