Mark all chromium test swarming task requests as idempotent.

scripts/slave/recipes/chromium.expected/full_client_skia_Mac10_7_Tests__1_.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "[BUILD]/scripts/slave/bot_update.py",
       "--master",
       "client.skia",
       "--builder",
       "Mac10.7 Tests (1)",
@@ skipping 202 matching lines @@
       "--tag",
       "master:client.skia",
       "--tag",
       "name:browser_tests",
       "--tag",
       "os:Mac-10.8",
       "--tag",
       "project:chromium",
       "--tag",
       "purpose:CI",
+      "--idempotent",
       "ffffffffffffffffffffffffffffffffffffffff",
       "--",
       "--test-launcher-summary-output=${ISOLATED_OUTDIR}/output.json"
     ],
     "name": "[trigger] browser_tests",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@json.output@{@@@",
       "@@@STEP_LOG_LINE@json.output@  \"base_task_name\": \"browser_tests/Mac-10.8/ffffffffff/Mac10.7 Tests (1)/571\", @@@",
       "@@@STEP_LOG_LINE@json.output@  \"tasks\": {@@@",
       "@@@STEP_LOG_LINE@json.output@    \"browser_tests/Mac-10.8/ffffffffff/Mac10.7 Tests (1)/571\": {@@@",
@@ skipping 113 matching lines @@
       "https://chromium-swarm.appspot.com",
       "--decorate",
       "--print-status-updates",
       "--shards",
       "1",
       "browser_tests/Mac-10.8/ffffffffff/Mac10.7 Tests (1)/571"
     ],
     "name": "browser_tests"
   }
 ]