Not sure what's causing this. Adding Infra>Platform>Swarming component for triage.

Failures were
"webkit_layout_tests on (none) GPU on Mac/Mac-10.11.6/..." for builders mac10.11_blink_rel, WebKit Mac10.11 (dbg), WebKit Mac10.11, Mac10.11 Tests so it wasn't due to a specific build configuration but it was for layout tests.

Not all tests timed out, only a subset. It's now succeeding: https://screenshot.googleplex.com/4esa09mYpBN

A few timeouts for posterity;
https://chromium-swarm.appspot.com/task?id=3ae62e75b163b510
https://chromium-swarm.appspot.com/task?id=3ae0c8c924a35b10
https://chromium-swarm.appspot.com/task?id=3adff75cb036fd10
https://chromium-swarm.appspot.com/task?id=3ade8d056459cc10
https://chromium-swarm.appspot.com/task?id=3adc0281c1de9910

I looked at all the tasks above and they are using exparchive, so isolated slowness is not the problem.

I'll dupe into issue 798828 since it's not a problem at the moment and we need 798828 to be fixed to understand where time was wasted.

That said, if this still impacts you please reopen. The number of timed out tasks is generally quite high;
https://chromium-swarm.appspot.com/tasklist?c=name&c=state&c=created_ts&c=duration&c=pending_time&c=pool&c=bot&f=state%3ATIMED_OUT&l=50&n=true&s=created_ts%3Adesc

Actually, it's probably a dupe of 794085. Reduping.