Example failing build:
https://build.chromium.org/p/chromium.gpu.fyi/builders/Win7%20Release%20%28NVIDIA%20GeForce%20730%29/builds/267

Error log excerpt:

1444 2016-03-31 03:50:30.905 I: Profiling: Section Setup took 1.226 seconds
1444 2016-03-31 03:50:33.470 I: Profiling: Section GetIsolateds took 2.561 seconds
1444 2016-03-31 03:50:33.703 I: Retrieving remaining files (0 of them)...
1444 2016-03-31 03:50:40.618 I: Profiling: Section GetRest took 7.148 seconds
1444 2016-03-31 03:50:40.688 I:     0 (       0kb) added
1444 2016-03-31 03:50:40.691 I: 12686 (  581175kb) current
1444 2016-03-31 03:50:40.691 I:     0 (       0kb) removed
1444 2016-03-31 03:50:40.693 I:        395969172kb free
1444 2016-03-31 03:50:40.693 I: Profiling: Section CleanupTrimming took 0.075 seconds
1444 2016-03-31 03:50:46.535 I: run_command(['C:\\b\\depot_tools\\python276_bin\\python.exe', u'..\\testing\\scripts\\run_telemetry_benchmark_as_googletest.py', u'../content/test/gpu/run_gpu_test.py', 'webgl_conformance', '--show-stdout', '--browser=release', '-v', '--extra-browser-args=--enable-logging=stderr --js-flags=--expose-gc', '--webgl-conformance-version=2.0.0', '--webgl2-only=true', '--isolated-script-test-output', 'c:\\users\\chrome~1.lab\\appdata\\local\\temp\\tmpl9izsn.json'], c:\users\chrome~1.lab\appdata\local\temp\isolated_run7ltyau\chrome)
(WARNING) 2016-03-30 20:50:47,687 desktop_browser_finder.FindAllAvailableBrowsers:148  Chrome build location for win_AMD64 not found. Browser will be run without Flash.
(INFO) 2016-03-30 20:50:47,687 browser_finder.FindBrowser:122  Chose browser: PossibleDesktopBrowser(type=release, executable=c:\users\chrome~1.lab\appdata\local\temp\isolated_run7ltyau\out\Release\chrome.exe, flash=None)
(INFO) 2016-03-30 20:50:47,717 exception_formatter.PrintFormattedException:20  Try printing formatted exception: <type 'exceptions.Exception'> The WebGL conformance test path specified inexpectation does not exist: c:\users\chrome~1.lab\appdata\local\temp\isolated_run7ltyau\third_party\webgl\src\sdk\tests\conformance2\textures\image_bitmap_from_image_bitmap\tex-image-and-sub-image-2d-with-image-bitmap-from-image-bitmap-r11f_g11f_b10f-rgb-unsigned_int_10f_11f_11f_rev.html <traceback object at 0x03BFCB70>

Traceback (most recent call last):
  <module> at content\test\gpu\run_gpu_test.py:92
    retcode = benchmark_runner.main(gpu_project_config.CONFIG)
  main at third_party\catapult\telemetry\telemetry\benchmark_runner.py:417
    return command_instance.Run(options)
  Run at third_party\catapult\telemetry\telemetry\benchmark_runner.py:236
    return min(255, self._benchmark().Run(args))
  Run at third_party\catapult\telemetry\telemetry\benchmark.py:91
    return story_runner.RunBenchmark(self, finder_options)
  RunBenchmark at third_party\catapult\telemetry\telemetry\internal\story_runner.py:298
    stories = benchmark.CreateStorySet(finder_options)
  CreateStorySet at content\test\gpu\gpu_tests\webgl_conformance.py:181
    expectations = self.GetExpectations()
  GetExpectations at content\test\gpu\gpu_tests\gpu_test_base.py:27
    self._cached_expectations = self._CreateExpectations()
  _CreateExpectations at content\test\gpu\gpu_tests\webgl_conformance.py:194
    conformance_path)
  __init__ at content\test\gpu\gpu_tests\webgl2_conformance_expectations.py:10
    super(WebGL2ConformanceExpectations, self).__init__(conformance_path)
  __init__ at content\test\gpu\gpu_tests\webgl_conformance_expectations.py:13
    super(WebGLConformanceExpectations, self).__init__()
  __init__ at content\test\gpu\gpu_tests\test_expectations.py:95
    self.SetExpectations()
  SetExpectations at content\test\gpu\gpu_tests\webgl2_conformance_expectations.py:512
    ['win7', ('nvidia', 0xf02)], bug= 483282 )
  Fail at content\test\gpu\gpu_tests\webgl_conformance_expectations.py:16
    self.CheckPatternIsValid(pattern)
  CheckPatternIsValid at content\test\gpu\gpu_tests\webgl_conformance_expectations.py:29
    'expectation does not exist: ' + full_path)
Exception: The WebGL conformance test path specified inexpectation does not exist: c:\users\chrome~1.lab\appdata\local\temp\isolated_run7ltyau\third_party\webgl\src\sdk\tests\conformance2\textures\image_bitmap_from_image_bitmap\tex-image-and-sub-image-2d-with-image-bitmap-from-image-bitmap-r11f_g11f_b10f-rgb-unsigned_int_10f_11f_11f_rev.html

Locals:
  full_path : 'c:\\users\\chrome~1.lab\\appdata\\local\\temp\\isolated_run7ltyau\\third_party\\webgl\\src\\sdk\\tests\\conformance2\\textures\\image_bitmap_from_image_bitmap\\tex-image-and-sub-image-2d-with-image-bitmap-from-image-bitmap-r11f_g11f_b10f-rgb-unsigned_int_10f_11f_11f_rev.html'
  pattern   : 'conformance2/textures/image_bitmap_from_image_bitmap/tex-image-and-sub-image-2d-with-image-bitmap-from-image-bitmap-r11f_g11f_b10f-rgb-unsigned_int_10f_11f_11f_rev.html'



This isn't reproducible locally. It surely has something to do with Swarming and how it sets up its virtual file system. I'm going to try a workaround which I unfortunately can't test.