Thanks for the report. There wasn't any evidence of maps_pixel_test failing in the last 200 builds on this tryserver:
https://luci-milo.appspot.com/buildbot/tryserver.chromium.mac/mac_chromium_rel_ng/?limit=200

Dug a little more deeply into your failed try run, and the bot in particular:

https://chromium-swarm.appspot.com/task?id=37e361e47e75c410&refresh=10&show_raw=1

It's clear that the bot that ran your tryjob as well as another one (the one that failed pixel_test a couple of times on mac_chromium_rel_ng) has some sort of misconfiguration and I've taken it out of the Swarming pool. Filed Issue 754372 to have these machines looked at.

I believe this will address this flakiness. Please reopen this bug if you see it again.