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

scripts/slave/recipes/chromium.expected/full_chromium_chromiumos_Linux_ChromiumOS_Tests__dbg__1_.json

Index: scripts/slave/recipes/chromium.expected/full_chromium_chromiumos_Linux_ChromiumOS_Tests__dbg__1_.json
diff --git a/scripts/slave/recipes/chromium.expected/full_chromium_chromiumos_Linux_ChromiumOS_Tests__dbg__1_.json b/scripts/slave/recipes/chromium.expected/full_chromium_chromiumos_Linux_ChromiumOS_Tests__dbg__1_.json
index ff84317c838ddd6b9e3c88c16ae0f9af10f4e394..52ce8c41334c1054826bf7c43787b5009d28e383 100644
--- a/scripts/slave/recipes/chromium.expected/full_chromium_chromiumos_Linux_ChromiumOS_Tests__dbg__1_.json
+++ b/scripts/slave/recipes/chromium.expected/full_chromium_chromiumos_Linux_ChromiumOS_Tests__dbg__1_.json
@@ -297,14 +297,14 @@
       "--tag",
       "slavename:TestSlavename",
       "--tag",
-      "stepname:browser_tests",
+      "stepname:browser_tests on Ubuntu-12.04",
       "--idempotent",
       "ffffffffffffffffffffffffffffffffffffffff",
       "--",
       "--test-launcher-summary-output=${ISOLATED_OUTDIR}/output.json"
     ],
     "cwd": "[SLAVE_BUILD]",
-    "name": "[trigger] browser_tests",
+    "name": "[trigger] browser_tests on Ubuntu-12.04",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@json.output@{@@@",
       "@@@STEP_LOG_LINE@json.output@  \"base_task_name\": \"browser_tests/Ubuntu-12.04/ffffffffff/Linux ChromiumOS Tests (dbg)(1)/571\", @@@",
@@ -343,7 +343,7 @@
       "browser_tests/Ubuntu-12.04/ffffffffff/Linux ChromiumOS Tests (dbg)(1)/571"
     ],
     "cwd": "[SLAVE_BUILD]",
-    "name": "browser_tests",
+    "name": "browser_tests on Ubuntu-12.04",
     "~followup_annotations": [
       "@@@STEP_LINK@shard #0 isolated out@blah@@@"
     ]
@@ -362,14 +362,14 @@
       "--build-number",
       "571",
       "--test-type",
-      "browser_tests",
+      "browser_tests on Ubuntu-12.04",
       "--test-results-server",
       "test-results.appspot.com",
       "--chrome-revision",
       "170242"
     ],
     "cwd": "[SLAVE_BUILD]",
-    "name": "Upload to test-results [browser_tests]"
+    "name": "Upload to test-results [browser_tests on Ubuntu-12.04]"
   },
   {
     "name": "$result",