buildbot: Enable multiple Mac configs on 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 12053 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-10.9"
+            },
+            {
+              "os": "Mac-10.12"
+            },
+            {
+              "gpu": "8086:0a2e",
+              "os": "Mac-10.12"
             }
           ],
           "shards": 8
         }
       }
     ]
   },
   "WebKit Win - RandomOrder": {
     "isolated_scripts": [
       {
@@ skipping 268 matching lines @@
         "test": "views_unittests"
       }
     ]
   },
   "Win Builder (ANGLE)": {
     "additional_compile_targets": [
       "all"
     ]
   }
 }