This CL: https://chromium-review.googlesource.com/c/chromium/src/+/1294811/5

Failed pixel_test on the first build, but succeeded on the second

The error was fairly surprising:
https://chromium-swarm.appspot.com/task?id=40bb4bd22aa5b310&refresh=10&show_raw=1

"""
82 files remaining...
Additional test environment:
    CHROME_DEVEL_SANDBOX=/opt/chromium/chrome_sandbox
    CHROME_HEADLESS=1
    LANG=en_US.UTF-8
Command: c:\b\s\w\ir\.swarming_module_cache\vpython\bf8312\Scripts\python.exe ..\..\testing\scripts\run_gpu_integration_test_as_googletest.py ../../content/test/gpu/run_gpu_integration_test.py pixel --show-stdout --browser=release --passthrough -v --extra-browser-args=--enable-logging=stderr --js-flags=--expose-gc --dont-restore-color-profile-after-test --refimg-cloud-storage-bucket chromium-gpu-archive/reference-images --os-type win --build-revision 2f969bff5532233afff5564f947b453b5dac6be6 --test-machine-name win7_chromium_rel_ng --download-refimg-from-cloud-storage --isolated-script-test-filter gpu_tests.pixel_integration_test.PixelIntegrationTest.Pixel_2DCanvasWebGL --isolated-script-test-output=c:\b\s\w\ioia_gvt\output.json --isolated-script-test-perf-output=c:\b\s\w\ioia_gvt\perftest-output.json

Traceback (most recent call last):
  File "..\..\testing\scripts\run_gpu_integration_test_as_googletest.py", line 67, in <module>
    sys.exit(main())
  File "..\..\testing\scripts\run_gpu_integration_test_as_googletest.py", line 50, in main
    return adapter.run_test()
  File "c:\b\s\w\ir\testing\scripts\common.py", line 334, in run_test
    cmd = self.generate_isolated_script_cmd()
  File "c:\b\s\w\ir\testing\scripts\common.py", line 300, in generate_isolated_script_cmd
    self.options.isolated_script_test_filter)
  File "c:\b\s\w\ir\testing\scripts\common.py", line 274, in generate_test_filter_args
    raise RuntimeError('this method is not yet implemented')
RuntimeError: this method is not yet implemented
"""

kbr -- any idea why this would happen?