Mark all chromium test swarming task requests as idempotent.

scripts/slave/recipes/chromium_trybot.expected/swarming_missing_isolated.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "[BUILD]/scripts/slave/bot_update.py",
       "--master",
       "tryserver.chromium.linux",
       "--builder",
       "linux_chromium_rel",
@@ skipping 263 matching lines @@
       "--tag",
       "os:Ubuntu-12.04",
       "--tag",
       "project:chromium",
       "--tag",
       "purpose:ManualTS",
       "--tag",
       "purpose:pre-commit",
       "--tag",
       "rietveld:https://codereview.chromium.org/12853011/#ps1",
+      "--idempotent",
       "[dummy hash for base_unittests]",
       "--",
       "--test-launcher-summary-output=${ISOLATED_OUTDIR}/output.json"
     ],
     "name": "[trigger] base_unittests (with patch)",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@json.output@{@@@",
       "@@@STEP_LOG_LINE@json.output@  \"base_task_name\": \"base_unittests (with patch)/Ubuntu-12.04/[dummy has/linux_chromium_rel/571\", @@@",
       "@@@STEP_LOG_LINE@json.output@  \"tasks\": {@@@",
       "@@@STEP_LOG_LINE@json.output@    \"base_unittests (with patch)/Ubuntu-12.04/[dummy has/linux_chromium_rel/571\": {@@@",
@@ skipping 196 matching lines @@
       "@@@STEP_TEXT@TEST RESULTS WERE INVALID@@@",
       "@@@STEP_FAILURE@@@"
     ]
   },
   {
     "name": "$final_result",
     "reason": "Step('browser_tests') failed with return_code 1",
     "status_code": 1
   }
 ]