v8.browsing_desktop failing on 2 builders

Builders failed on: 
- Mac Air 10.11 Perf: 
  https://build.chromium.org/p/chromium.perf/builders/Mac%20Air%2010.11%20Perf
- Mac Pro 10.11 Perf: 
  https://build.chromium.org/p/chromium.perf/builders/Mac%20Pro%2010.11%20Perf


Looks to be failing just on mac. Seeing lots of "ValueError: No JSON object could be decoded":

https://luci-logdog.appspot.com/v/?s=chrome%2Fbb%2Fchromium.perf%2FMac_Air_10.11_Perf%2F1151%2F%2B%2Frecipes%2Fsteps%2Fv8.browsing_desktop_on_Intel_GPU_on_Mac_on_Mac-10.11%2F0%2Fstdout
Traceback (most recent call last):
  File "/b/rr/tmpG5V63L/rw/checkout/scripts/slave/recipe_modules/swarming/resources/standard_isolated_script_merge.py", line 45, in <module>
    sys.exit(main())
  File "/b/rr/tmpG5V63L/rw/checkout/scripts/slave/recipe_modules/swarming/resources/standard_isolated_script_merge.py", line 41, in main
    return StandardIsolatedScriptMerge(args.output_json, args.jsons_to_merge)
  File "/b/rr/tmpG5V63L/rw/checkout/scripts/slave/recipe_modules/swarming/resources/standard_isolated_script_merge.py", line 24, in StandardIsolatedScriptMerge
    shard_results_list.append(json.load(f))
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/json/__init__.py", line 290, in load
    **kw)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/json/__init__.py", line 338, in loads
    return _default_decoder.decode(s)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/json/decoder.py", line 366, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/json/decoder.py", line 384, in raw_decode
    raise ValueError("No JSON object could be decoded")
ValueError: No JSON object could be decoded
WARNING:root:merge_cmd had non-zero return code: 1
step returned non-zero exit code: 241

Since this is in swarming, Emily or Stephen can you help triage?