Added swarming step to the Blink trybots

scripts/slave/recipes/chromium.expected/full_chromium_win_Vista_Tests__1_.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "[BUILD]\\scripts\\slave\\kill_processes.py"
     ],
     "name": "taskkill"
   },
   {
@@ skipping 224 matching lines @@
       "--tag",
       "master:chromium.win",
       "--tag",
       "name:browser_tests",
       "--tag",
       "os:Windows-6.0",
       "--tag",
       "project:chromium",
       "--tag",
       "purpose:CI",
+      "--tag",
+      "purpose:post-commit",
       "--idempotent",
       "ffffffffffffffffffffffffffffffffffffffff",
       "--",
       "--test-launcher-summary-output=${ISOLATED_OUTDIR}/output.json"
     ],
     "name": "[trigger] browser_tests on Windows-6.0",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@json.output@{@@@",
       "@@@STEP_LOG_LINE@json.output@  \"base_task_name\": \"browser_tests/Windows-6.0/ffffffffff/Vista Tests (1)/571\", @@@",
       "@@@STEP_LOG_LINE@json.output@  \"tasks\": {@@@",
@@ skipping 108 matching lines @@
     "cmd": [
       "python",
       "-u",
       "[BUILD]\\scripts\\slave\\process_dumps.py",
       "--target",
       "Release"
     ],
     "name": "process_dumps"
   }
 ]