https://stainless.corp.google.com/browse/chromeos-autotest-results/253877354-chromeos-test/

Traceback (most recent call last):
  File "/usr/local/autotest/common_lib/test.py", line 600, in _exec
    _call_test_function(self.execute, *p_args, **p_dargs)
  File "/usr/local/autotest/common_lib/test.py", line 806, in _call_test_function
    raise error.UnhandledTestFail(e)
UnhandledTestFail: Unhandled TypeError: unsupported operand type(s) for -: 'str' and 'float'
Traceback (most recent call last):
  File "/usr/local/autotest/common_lib/test.py", line 800, in _call_test_function
    return func(*args, **dargs)
  File "/usr/local/autotest/common_lib/test.py", line 464, in execute
    postprocess_profiled_run, args, dargs)
  File "/usr/local/autotest/common_lib/test.py", line 371, in _call_run_once
    self.run_once(*args, **dargs)
  File "/usr/local/autotest/tests/power_Resume/power_Resume.py", line 41, in run_once
    measure_arc)
  File "/usr/local/autotest/tests/power_Resume/power_Resume.py", line 53, in _suspend_once
    measure_arc=measure_arc)
  File "/usr/local/autotest/cros/power/power_suspend.py", line 602, in suspend
    self._arc_resume_ts() - start_resume
TypeError: unsupported operand type(s) for -: 'str' and 'float'

Error is captured in power_UIResume.ERROR,

10/31 07:47:12.393 ERROR|     power_suspend:0489| ARC did not resume correctly.

but should handle problem w/ self._arc_resume_ts not being a valid timestamp.