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

scripts/slave/recipes/v8.expected/full_client_v8_V8_Win32___nosnap___shared.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "[BUILD]\\scripts\\slave\\kill_processes.py"
     ],
     "cwd": "[SLAVE_BUILD]",
     "name": "taskkill"
   },
@@ skipping 200 matching lines @@
       "os:Windows-7-SP1",
       "--tag",
       "project:v8",
       "--tag",
       "purpose:CI",
       "--tag",
       "purpose:post-commit",
       "--tag",
       "slavename:TestSlavename",
       "--tag",
-      "stepname:Check",
+      "stepname:Check on Windows-7-SP1",
       "--idempotent",
       "[dummy hash for bot_default]",
       "--",
       "--progress=verbose",
       "--mode",
       "Release",
       "--arch",
       "ia32",
       "--outdir",
       "out",
       "--buildbot",
       "--timeout=200",
       "--random-seed=-976563914",
       "bot_default",
       "--exhaustive-variants",
       "--no-snap",
       "--rerun-failures-count=2",
       "--swarming",
       "--json-test-results",
       "${ISOLATED_OUTDIR}/output.json"
     ],
     "cwd": "[SLAVE_BUILD]",
-    "name": "[trigger] Check",
+    "name": "[trigger] Check on Windows-7-SP1",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@json.output@{@@@",
       "@@@STEP_LOG_LINE@json.output@  \"base_task_name\": \"Check/Windows-7-SP1/[dummy has/V8 Win32 - nosnap - shared/571\", @@@",
       "@@@STEP_LOG_LINE@json.output@  \"tasks\": {@@@",
       "@@@STEP_LOG_LINE@json.output@    \"Check/Windows-7-SP1/[dummy has/V8 Win32 - nosnap - shared/571\": {@@@",
       "@@@STEP_LOG_LINE@json.output@      \"shard_index\": 0, @@@",
       "@@@STEP_LOG_LINE@json.output@      \"task_id\": \"10000\", @@@",
       "@@@STEP_LOG_LINE@json.output@      \"view_url\": \"https://chromium-swarm.appspot.com/user/task/10000\"@@@",
       "@@@STEP_LOG_LINE@json.output@    }@@@",
       "@@@STEP_LOG_LINE@json.output@  }@@@",
@@ skipping 42 matching lines @@
       "@@@STEP_LOG_LINE@durations@Duration: 00:00:101@@@",
       "@@@STEP_LOG_END@durations@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]