Turn off swarming for WebKit Mac10.12.

BUG=697971

Review-Url: https://codereview.chromium.org/2738933002
Cr-Commit-Position: refs/heads/master@{#455776}

testing/buildbot/chromium.webkit.json

@@ -220,27 +220,15 @@
   "WebKit Mac10.12": {
     "gtest_tests": [
       {
-        "swarming": {
-          "can_use_on_swarming_builders": true
-        },
         "test": "blink_heap_unittests"
       },
       {
-        "swarming": {
-          "can_use_on_swarming_builders": true
-        },
         "test": "blink_platform_unittests"
       },
       {
-        "swarming": {
-          "can_use_on_swarming_builders": true
-        },
         "test": "webkit_unit_tests"
       },
       {
-        "swarming": {
-          "can_use_on_swarming_builders": true
-        },
         "test": "wtf_unittests"
       }
     ],