From most recent build: 
https://ci.chromium.org/p/chrome/builders/luci.chrome.ci/Android%20Nexus5X%20WebView%20Perf/3521


You can see that all shards are failing for nexus 5x webview.  
 
Looks like a problem finding the browser: 

https://chrome-swarming.appspot.com/task?id=410d713c51324f10&refresh=10&show_raw=1

(INFO) 2018-11-08 18:40:41,034 exception_formatter.PrintFormattedException:19  Try printing formatted exception: <class 'telemetry.core.exceptions.PathMissingError'> Unable to find apk specified by --webview-embedder-apk=None <traceback object at 0x7f6ac1dd7368>

Traceback (most recent call last):
  <module> at /b/swarming/w/ir/tools/perf/run_benchmark:27
    sys.exit(main())
  main at /b/swarming/w/ir/tools/perf/run_benchmark:23
    return benchmark_runner.main(config, [trybot_command.Trybot])
  main at /b/swarming/w/ir/third_party/catapult/telemetry/telemetry/benchmark_runner.py:446
    return command_instance.Run(options)
  Run at /b/swarming/w/ir/third_party/catapult/telemetry/telemetry/benchmark_runner.py:316
    return min(255, b.Run(args))
  Run at /b/swarming/w/ir/third_party/catapult/telemetry/telemetry/benchmark.py:105
    return story_runner.RunBenchmark(self, finder_options)
  RunBenchmark at /b/swarming/w/ir/third_party/catapult/telemetry/telemetry/internal/story_runner.py:314
    benchmark.CustomizeBrowserOptions(finder_options.browser_options)
  CustomizeBrowserOptions at /b/swarming/w/ir/tools/perf/core/perf_benchmark.py:96
    variations = self._GetVariationsBrowserArgs(options.finder_options)
  _GetVariationsBrowserArgs at /b/swarming/w/ir/tools/perf/core/perf_benchmark.py:141
  Traceback (most recent call last):
  File "/b/swarming/w/ir/testing/scripts/run_telemetry_benchmark_as_googletest.py", line 153, in run_benchmark
    with open(tempfile_name) as f:
IOError: [Errno 2] No such file or directory: '/b/swarming/w/it3epPNe/tmpbxLz4ltelemetry/histograms.json'
  possible_browser = browser_finder.FindBrowser(finder_options)
  Cacher at /b/swarming/w/ir/third_party/catapult/telemetry/telemetry/decorators.py:35
    cacher.__cache[key] = obj(*args, **kwargs)
  FindBrowser at /b/swarming/w/ir/third_party/catapult/telemetry/telemetry/internal/browser/browser_finder.py:69
    curr_browsers = finder.FindAllAvailableBrowsers(options, device)
  FindAllAvailableBrowsers at /b/swarming/w/ir/third_party/catapult/telemetry/telemetry/internal/backends/chrome/android_browser_finder.py:400
    return _FindAllPossibleBrowsers(finder_options, android_platform)
  _FindAllPossibleBrowsers at /b/swarming/w/ir/third_party/catapult/telemetry/telemetry/internal/backends/chrome/android_browser_finder.py:334
    finder_options.browser_executable)
PathMissingError: Unable to find apk specified by --webview-embedder-apk=None

I would bet the root cause is probably the same as  issue 903425 .