swarming: always append task's OS dimension to step name

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

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[depot_tools::bot_update]/resources/bot_update.py",
       "--master",
       "chromium.linux",
       "--builder",
       "Linux Tests",
@@ skipping 283 matching lines @@
       "os:Ubuntu-12.04",
       "--tag",
       "project:chromium",
       "--tag",
       "purpose:CI",
       "--tag",
       "purpose:post-commit",
       "--tag",
       "slavename:TestSlavename",
       "--tag",
-      "stepname:telemetry_gpu_unittests",
+      "stepname:telemetry_gpu_unittests on Ubuntu-12.04",
       "eeeeeeeeeeeeeeeeeeeeeeeeeeeeee",
       "--",
       "--isolated-script-test-output=${ISOLATED_OUTDIR}/output.json"
     ],
     "cwd": "[SLAVE_BUILD]",
-    "name": "[trigger] telemetry_gpu_unittests",
+    "name": "[trigger] telemetry_gpu_unittests on Ubuntu-12.04",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@json.output@{@@@",
       "@@@STEP_LOG_LINE@json.output@  \"base_task_name\": \"telemetry_gpu_unittests/Ubuntu-12.04/eeeeeeeeee/Linux Tests/571\", @@@",
       "@@@STEP_LOG_LINE@json.output@  \"tasks\": {@@@",
       "@@@STEP_LOG_LINE@json.output@    \"telemetry_gpu_unittests/Ubuntu-12.04/eeeeeeeeee/Linux Tests/571\": {@@@",
       "@@@STEP_LOG_LINE@json.output@      \"shard_index\": 0, @@@",
       "@@@STEP_LOG_LINE@json.output@      \"task_id\": \"10000\", @@@",
       "@@@STEP_LOG_LINE@json.output@      \"view_url\": \"https://chromium-swarm.appspot.com/user/task/10000\"@@@",
       "@@@STEP_LOG_LINE@json.output@    }@@@",
       "@@@STEP_LOG_LINE@json.output@  }@@@",
@@ skipping 12 matching lines @@
       "https://chromium-swarm.appspot.com",
       "--decorate",
       "--print-status-updates",
       "--shards",
       "1",
       "telemetry_gpu_unittests/Ubuntu-12.04/eeeeeeeeee/Linux Tests/571",
       "--task-output-dir",
       "/path/to/tmp/"
     ],
     "cwd": "[SLAVE_BUILD]",
-    "name": "telemetry_gpu_unittests",
+    "name": "telemetry_gpu_unittests on Ubuntu-12.04",
     "~followup_annotations": [
       "step returned non-zero exit code: 255",
       "@@@STEP_TEXT@<br/>failures:<br/>test1.Test1<br/>test2.Test2<br/>@@@",
       "@@@STEP_LOG_LINE@outdir_json@{@@@",
       "@@@STEP_LOG_LINE@outdir_json@  \"0/output.json\": \"{\\\"failures\\\": [\\\"test1.Test1\\\", \\\"test2.Test2\\\"], \\\"valid\\\": true}\", @@@",
       "@@@STEP_LOG_LINE@outdir_json@  \"summary.json\": \"{\\\"shards\\\": [{\\\"internal_failure\\\": false, \\\"failure\\\": false}]}\"@@@",
       "@@@STEP_LOG_LINE@outdir_json@}@@@",
       "@@@STEP_LOG_END@outdir_json@@@",
       "@@@STEP_FAILURE@@@"
     ]
   },
   {
     "name": "$result",
     "reason": "1 tests failed: ['telemetry_gpu_unittests']",
     "status_code": 1
   }
 ]