Happening consistently as of:
https://luci-milo.appspot.com/buildbot/chromium.perf/Android%20Nexus5X%20WebView%20Perf%20%282%29/4890

Error log looks like:

ERROR:root:Git hash returned from BattOr was not as expected: [0411/163848.622203:ERROR:serial_io_handler.cc(152)] Failed to open serial port: FILE_ERROR_ACCESS_DENIED
Traceback (most recent call last):
  File "/b/c/b/Android_Nexus5X_WebView_Perf__2_/src/third_party/catapult/common/battor/battor/battor_wrapper.py", line 145, in _FlashBattOr
    device_git_hash = self.GetFirmwareGitHash()
  File "/b/c/b/Android_Nexus5X_WebView_Perf__2_/src/third_party/catapult/common/battor/battor/battor_wrapper.py", line 373, in GetFirmwareGitHash
    int(self._git_hash, 16)
ValueError: invalid literal for int() with base 16: '[0411/163848.622203:ERROR:serial_io_handler.cc(152)] Failed to open serial port: FILE_ERROR_ACCESS_DENIED'
CRITICAL:root:BattOr shell was not properly closed. Killing now.
[...]
INFO:devil.utils.cmd_helper:[host]> /b/c/b/Android_Nexus5X_WebView_Perf__2_/src/third_party/catapult/devil/bin/deps/linux2/x86_64/bin/adb -s 02020871c2a35531 shell 'rm -f /data/local/tmp/temp_file-3d927b051c224.png'
Traceback (most recent call last):
  File "/b/c/b/Android_Nexus5X_WebView_Perf__2_/src/third_party/catapult/telemetry/telemetry/internal/story_runner.py", line 81, in _RunStoryAndProcessErrorIfNeeded
    test.WillRunStory(state.platform)
  File "/b/c/b/Android_Nexus5X_WebView_Perf__2_/src/third_party/catapult/telemetry/telemetry/web_perf/timeline_based_measurement.py", line 285, in WillRunStory
    platform.tracing_controller.StartTracing(self._tbm_options.config)
  File "/b/c/b/Android_Nexus5X_WebView_Perf__2_/src/third_party/catapult/telemetry/telemetry/core/tracing_controller.py", line 43, in StartTracing
    self._tracing_controller_backend.StartTracing(tracing_config, timeout)
  File "/b/c/b/Android_Nexus5X_WebView_Perf__2_/src/third_party/catapult/telemetry/telemetry/internal/platform/tracing_controller_backend.py", line 91, in StartTracing
    started = agent.StartAgentTracing(config, timeout)
  File "/b/c/b/Android_Nexus5X_WebView_Perf__2_/src/third_party/catapult/telemetry/telemetry/internal/platform/tracing_agent/battor_tracing_agent.py", line 73, in StartAgentTracing
    self._battor.StartTracing()
  File "/b/c/b/Android_Nexus5X_WebView_Perf__2_/src/third_party/catapult/common/battor/battor/battor_wrapper.py", line 216, in StartTracing
    self._SendBattOrCommand(self._START_TRACING_CMD)
  File "/b/c/b/Android_Nexus5X_WebView_Perf__2_/src/third_party/catapult/common/battor/battor/battor_wrapper.py", line 334, in _SendBattOrCommand
    'Outputted: %s' % (cmd, status))
BattOrError: BattOr did not complete command 'StartTracing' correctly.
Outputted: [0411/163848.622265:FATAL:battor_agent_bin.cc(89)] Fatal error when communicating with the BattOr: CONNECTION FAILED
https://luci-logdog.appspot.com/v/?s=chrome%2Fbb%2Fchromium.perf%2FAndroid_Nexus5X_WebView_Perf__2_%2F4892%2F%2B%2Frecipes%2Fsteps%2Fsystem_health.common_mobile%2F0%2Fstdout