Labeling bisect/perf try jobs with a prominent null step.

scripts/slave/recipes/bisection/desktop_bisect.expected/basic.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[depot_tools::bot_update]/resources/bot_update.py",
       "--master",
       "tryserver.chromium.perf",
       "--builder",
       "linux_perf_bisect",
@@ skipping 177 matching lines @@
     ],
     "cwd": "[SLAVE_BUILD]/src",
     "name": "git diff to analyze patch",
     "stdout": "/path/to/tmp/",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@files@tools/auto_bisect/bisect.cfg@@@",
       "@@@STEP_LOG_END@files@@@"
     ]
   },
   {
+    "cmd": [],
+    "name": "***LEGACY BISECT (deprecated)***"
+  },
+  {
     "cmd": [
       "python",
       "-u",
       "[SLAVE_BUILD]/src/tools/prepare-bisect-perf-regression.py",
       "-w",
       "[SLAVE_BUILD]"
     ],
     "name": "Preparing for Bisection"
   },
   {
@@ skipping 23 matching lines @@
       "{\"blamelist\": [\"cool_dev1337@chromium.org\", \"hax@chromium.org\"], \"branch\": \"\", \"buildername\": \"linux_perf_bisect\", \"buildnumber\": 571, \"issue\": 12853011, \"mastername\": \"tryserver.chromium.perf\", \"patch_project\": \"chromium\", \"patchset\": 1, \"project\": \"chrome\", \"recipe\": \"bisection/desktop_bisect\", \"repository\": \"\", \"requester\": \"commit-bot@chromium.org\", \"revision\": \"HEAD\", \"rietveld\": \"https://codereview.chromium.org\", \"slavename\": \"TestSlavename\", \"workdir\": \"/path/to/workdir/TestSlavename\"}"
     ],
     "name": "Running Bisection"
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]