"webkit_layout_tests (without patch)" is failing on linux_layout_tests_slimming_paint_v2 with TEST RESULTS WERE INVALID:

https://logs.chromium.org/logs/chromium/buildbucket/cr-buildbucket.appspot.com/8932573215890582176/+/steps/webkit_layout_tests__without_patch_/0/stdout

Some snippet from the stdout:

  Set PYTHONPATH: /b/swarming/w/ir/kitchen-checkout/build/scripts:/b/swarming/w/ir/kitchen-checkout/build/site_config:/b/swarming/w/ir/kitchen-checkout/build/third_party:/b/swarming/w/ir/kitchen-checkout/build/third_party/buildbot_8_4p1:/b/swarming/w/ir/kitchen-checkout/build/third_party/buildbot_slave_8_4:/b/swarming/w/ir/kitchen-checkout/build/third_party/coverage-3.7.1:/b/swarming/w/ir/kitchen-checkout/build/third_party/decorator_3_3_1:/b/swarming/w/ir/kitchen-checkout/build/third_party/google_api_python_client:/b/swarming/w/ir/kitchen-checkout/build/third_party/httplib2/python2:/b/swarming/w/ir/kitchen-checkout/build/third_party/infra_libs:/b/swarming/w/ir/kitchen-checkout/build/third_party/jinja2:/b/swarming/w/ir/kitchen-checkout/build/third_party/markupsafe:/b/swarming/w/ir/kitchen-checkout/build/third_party/oauth2client:/b/swarming/w/ir/kitchen-checkout/build/third_party/pyasn1:/b/swarming/w/ir/kitchen-checkout/build/third_party/pyasn1-modules:/b/swarming/w/ir/kitchen-checkout/build/third_party/python-rsa:/b/swarming/w/ir/kitchen-checkout/build/third_party/requests_2_10_0:/b/swarming/w/ir/kitchen-checkout/build/third_party/setuptools-0.6c11:/b/swarming/w/ir/kitchen-checkout/build/third_party/sqlalchemy_0_7_1:/b/swarming/w/ir/kitchen-checkout/build/third_party/sqlalchemy_migrate_0_7_1:/b/swarming/w/ir/kitchen-checkout/build/third_party/tempita_0_5:/b/swarming/w/ir/kitchen-checkout/build/third_party/twisted_10_2:/b/swarming/w/ir/kitchen-checkout/build/third_party/uritemplate:/b/swarming/w/ir/kitchen-checkout/build/third_party/site-packages
  WARNING:root:task_output_dir '/b/swarming/w/ir/tmp/t/tmpFK2Fcl' already exists!
  WARNING:root:task_output_dir existing content: []
  +-----------------------------------------------------------------------------------+
  | Shard 0  https://chromium-swarm.appspot.com/user/task/409215c65edb2010            |
  +-----------------------------------------------------------------------------------+
  42 files remaining...
  Additional test environment:
      CHROME_DEVEL_SANDBOX=/opt/chromium/chrome_sandbox
      CHROME_HEADLESS=1
      LANG=en_US.UTF-8
  Command: /b/s/w/ir/.swarming_module_cache/vpython/dba151/bin/python ../../testing/scripts/run_isolated_script_test.py --xvfb ../../third_party/blink/tools/run_web_tests.py --seed 4 --no-show-results --zero-tests-executed-ok --clobber-old-results --exit-after-n-failures 5000 --exit-after-n-crashes-or-timeouts 100 --debug-rwt-logging --results-directory /b/s/w/iojKinKr/layout-test-results --num-retries=3 --additional-driver-flag=--enable-slimming-paint-v2 --isolated-script-test-filter compositing/force-compositing-mode/no-overflow-iframe-layer.html::compositing/force-compositing-mode/overflow-hidden-iframe-layer.html::virtual/paint-touchaction-rects/fast/events/touch/compositor-touch-hit-rects-animation.html::virtual/paint-touchaction-rects/fast/events/touch/compositor-touch-hit-rects-global.html::virtual/paint-touchaction-rects/fast/events/touch/compositor-touch-hit-rects-many.html::virtual/paint-touchaction-rects/fast/events/touch/compositor-touch-hit-rects-non-composited-scroll-overflow-with-handler.html::virtual/paint-touchaction-rects/fast/events/touch/compositor-touch-hit-rects-non-composited-scroll.html::virtual/paint-touchaction-rects/fast/events/touch/compositor-touch-hit-rects-scroll.html::virtual/paint-touchaction-rects/fast/events/touch/compositor-touch-hit-rects-squashing.html::virtual/paint-touchaction-rects/fast/events/touch/compositor-touch-hit-rects-transform-changed-nolayout.html::virtual/paint-touchaction-rects/fast/events/touch/compositor-touch-hit-rects-trigger-commit.html::virtual/paint-touchaction-rects/fast/events/touch/compositor-touch-hit-rects-with-negative-child.html::virtual/paint-touchaction-rects/fast/events/touch/gesture/gesture-scroll-by-page.html::virtual/paint-touchaction-rects/fast/events/touch/gesture/touch-gesture-scroll-div-scaled.html::virtual/paint-touchaction-rects/fast/events/touch/page-scaled-touch-gesture-click.html::virtual/paint-touchaction-rects/fast/events/touch/touch-handler-assert-input-range.html::virtual/paint-touchaction-rects/fast/events/touch/touch-rect-assert-first-layer-special.html::virtual/paint-touchaction-rects/fast/events/touch/touch-rect-crash-on-unpromote-layer.html::virtual/paint-touchaction-rects/fast/events/touch/touch-scaled-scrolled.html --isolated-script-test-output=/b/s/w/iojKinKr/output.json --isolated-script-test-perf-output=/b/s/w/iojKinKr/perftest-output.json
  Traceback (most recent call last):
    File "../../testing/scripts/run_isolated_script_test.py", line 104, in <module>
      sys.exit(main())
    File "../../testing/scripts/run_isolated_script_test.py", line 88, in main
      adapter.run_test()
    File "../../testing/scripts/run_isolated_script_test.py", line 82, in run_test
      super(TypUnittestAdapter, self).run_test()
    File "/b/s/w/ir/testing/scripts/common.py", line 336, in run_test
      cmd = self.generate_isolated_script_cmd()
    File "/b/s/w/ir/testing/scripts/common.py", line 301, in generate_isolated_script_cmd
      self.options.isolated_script_test_filter)
    File "../../testing/scripts/run_isolated_script_test.py", line 77, in generate_test_filter_args
      return ['--%s=' % arg_name + self._temp_filter_file]
  TypeError: cannot concatenate 'str' and 'instance' objects