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

scripts/slave/recipes/chromium.expected/full_chromium_webkit_WebKit_Mac10_7.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[depot_tools::bot_update]/resources/bot_update.py",
       "--master",
       "chromium.webkit",
       "--builder",
       "WebKit Mac10.7",
@@ skipping 282 matching lines @@
       "os:Mac-10.8",
       "--tag",
       "project:chromium",
       "--tag",
       "purpose:CI",
       "--tag",
       "purpose:post-commit",
       "--tag",
       "slavename:TestSlavename",
       "--tag",
-      "stepname:browser_tests",
+      "stepname:browser_tests on Mac-10.8",
       "--idempotent",
       "ffffffffffffffffffffffffffffffffffffffff",
       "--",
       "--test-launcher-summary-output=${ISOLATED_OUTDIR}/output.json"
     ],
     "cwd": "[SLAVE_BUILD]",
-    "name": "[trigger] browser_tests",
+    "name": "[trigger] browser_tests on Mac-10.8",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@json.output@{@@@",
       "@@@STEP_LOG_LINE@json.output@  \"base_task_name\": \"browser_tests/Mac-10.8/ffffffffff/WebKit Mac10.7/571\", @@@",
       "@@@STEP_LOG_LINE@json.output@  \"tasks\": {@@@",
       "@@@STEP_LOG_LINE@json.output@    \"browser_tests/Mac-10.8/ffffffffff/WebKit Mac10.7/571\": {@@@",
       "@@@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 81 matching lines @@
       "collect",
       "--swarming",
       "https://chromium-swarm.appspot.com",
       "--decorate",
       "--print-status-updates",
       "--shards",
       "1",
       "browser_tests/Mac-10.8/ffffffffff/WebKit Mac10.7/571"
     ],
     "cwd": "[SLAVE_BUILD]",
-    "name": "browser_tests",
+    "name": "browser_tests on Mac-10.8",
     "~followup_annotations": [
       "@@@STEP_LINK@shard #0 isolated out@blah@@@"
     ]
   },
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[build::test_results]/resources/upload_gtest_test_results.py",
       "--input-gtest-json",
       "{\"per_iteration_data\": [{\"Test.One\": [{\"elapsed_time_ms\": 0, \"output_snippet\": \"\", \"status\": \"SUCCESS\"}], \"Test.Three\": [{\"elapsed_time_ms\": 0, \"output_snippet\": \"\", \"status\": \"SUCCESS\"}], \"Test.Two\": [{\"elapsed_time_ms\": 0, \"output_snippet\": \"\", \"status\": \"SUCCESS\"}]}], \"swarming_summary\": {\"shards\": [{\"outputs_ref\": {\"view_url\": \"blah\"}}]}}",
       "--master-name",
       "chromium.webkit",
       "--builder-name",
       "WebKit Mac10.7",
       "--build-number",
       "571",
       "--test-type",
-      "browser_tests",
+      "browser_tests on Mac-10.8",
       "--test-results-server",
       "test-results.appspot.com",
       "--chrome-revision",
       "170242"
     ],
     "cwd": "[SLAVE_BUILD]",
-    "name": "Upload to test-results [browser_tests]"
+    "name": "Upload to test-results [browser_tests on Mac-10.8]"
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]