Setting --run-python-script flag for runtest.py invocation.

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

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[depot_tools::bot_update]/resources/bot_update.py",
       "--master",
       "tryserver.chromium.perf",
       "--builder",
       "linux_perf_bisect",
@@ skipping 310 matching lines @@
       "Release",
       "--xvfb",
       "--factory-properties",
       "{\"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\"}",
       "--build-properties",
       "{\"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\"}",
       "--step-name=Performance Test (With Patch) 1 of 1",
       "--builder-name=linux_perf_bisect",
       "--slave-name=TestSlavename",
       "--build-number=571",
+      "--run-python-script",
       "../../../src/tools/perf/run_benchmark",
       "-v",
       "--browser=release",
       "sunspider",
       "--reset-results",
       "--upload-results",
       "--results-label=Patch"
     ],
     "cwd": "[SLAVE_BUILD]",
     "name": "Performance Test (With Patch) 1 of 1",
@@ skipping 201 matching lines @@
       "Release",
       "--xvfb",
       "--factory-properties",
       "{\"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\"}",
       "--build-properties",
       "{\"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\"}",
       "--step-name=Performance Test (Without Patch) 1 of 1",
       "--builder-name=linux_perf_bisect",
       "--slave-name=TestSlavename",
       "--build-number=571",
+      "--run-python-script",
       "../../../src/tools/perf/run_benchmark",
       "-v",
       "--browser=release",
       "sunspider",
       "--upload-results",
       "--results-label=TOT"
     ],
     "cwd": "[SLAVE_BUILD]",
     "name": "Performance Test (Without Patch) 1 of 1",
     "stderr": "/path/to/tmp/",
@@ skipping 43 matching lines @@
       "@@@STEP_LOG_LINE@json.output@}@@@",
       "@@@STEP_LOG_END@json.output@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]