Look at
https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/Linux%20Tests%20%28dbg%29%281%29%2832%29?limit=200

Notice that about every day or so, net_unittests fail in a series of consecutive runs for about five hours.  (This is usually about four runs.)

This is strange.

It was already happening by 2018-06-06.

It looks like a timeout issue.  I sampled six failures, at different points in the cycle, and they all report
"Test runtime exceeded allocated time"
and stop after 1 hour.

I look at four successful runs, and they all finished at 59 minutes and change, just under the time.

I don't know why the passes happen in series and the failures happen in series (maybe our load is cyclical?).  It's clear the timeout here is set too aggressively.