Inspection of the Mac tryserver:
https://luci-milo.appspot.com/buildbot/tryserver.chromium.mac/mac_chromium_rel_ng/?limit=200

and Mac Intel waterfall bot:
https://luci-milo.appspot.com/buildbot/chromium.gpu/Mac%20Release%20%28Intel%29/?limit=200

indicates flakiness of the screenshot_sync_tests, maps_pixel_test and pixel_test steps.

Digging deeper it strongly appears that the following two machines have started failing these tests 100% of the time:

https://chromium-swarm.appspot.com/bot?id=build737-m4&sort_stats=total%3Adesc
https://chromium-swarm.appspot.com/bot?id=build741-m4&sort_stats=total%3Adesc

When this has happened in the past, the IP KVM connection was broken and the logs started indicating that the vendor and device PCI IDs were 0. This isn't the case this time.

Regardless, it seems clear that these two machines are at fault. I'm going to take them out of the Swarming pool and file a Labs ticket to have them looked at.