e.g. https://luci-milo.appspot.com/buildbot/chromium.infra.cron/wpt-importer/5297, whose complete git-cl upload failure looks like this:

Running presubmit upload checks ...
Traceback (most recent call last):
  File "/data/src/depot_tools/git_cl.py", line 6126, in <module>
    sys.exit(main(sys.argv[1:]))
  File "/data/src/depot_tools/git_cl.py", line 6108, in main
    return dispatcher.execute(OptionParser(), argv)
  File "/data/src/depot_tools/subcommand.py", line 252, in execute
    return command(parser, args[1:])
  File "/data/src/depot_tools/git_cl.py", line 4764, in CMDpresubmit
    change=cl.GetChange(base_branch, None))
  File "/data/src/depot_tools/git_cl.py", line 1576, in RunHook
    gerrit_obj=self._codereview_impl.GetGerritObjForPresubmit())
  File "/data/src/depot_tools/presubmit_support.py", line 1403, in DoPresubmitChecks
    results += executer.ExecPresubmitScript(presubmit_script, filename)
  File "/data/src/depot_tools/presubmit_support.py", line 1311, in ExecPresubmitScript
    result = eval(function_name + '(*__args)', context)
  File "<string>", line 1, in <module>
  File "<string>", line 2716, in CheckChangeOnUpload
  File "<string>", line 2474, in _CommonChecks
  File "<string>", line 1592, in _CheckIpcOwners
  File "/usr/lib64/python2.7/json/__init__.py", line 339, in loads
    return _default_decoder.decode(s)
  File "/usr/lib64/python2.7/json/decoder.py", line 364, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/usr/lib64/python2.7/json/decoder.py", line 382, in raw_decode
    raise ValueError("No JSON object could be decoded")
ValueError: No JSON object could be decoded

The faulty file in question is third_party/WebKit/LayoutTests/external/wpt/XMLHttpRequest/resources/utf16-bom.json, from https://github.com/w3c/web-platform-tests/commit/678d58cbcc7491105b316d4d87b5675b3887bfe3