This is similar to  issue 840988 . First failing builds:

https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/Win10%20FYI%20Release%20%28NVIDIA%29/1142
https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/Win7%20FYI%20Release%20%28AMD%29/1241
https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/Win7%20FYI%20Release%20%28NVIDIA%29/1013
https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/Win7%20FYI%20x64%20Release%20%28NVIDIA%29/1071

Also affecting Mac, though not as flaky

https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/Mac%20Pro%20FYI%20Release%20(AMD)/1199
https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/Mac%20FYI%20Experimental%20Retina%20Release%20(NVIDIA)/611

Unsure if it affects Linux.

Errors seem to be similar, a bunch of warnings like this:

[7332:3892:0519/170013.751:WARNING:gaia_auth_fetcher.cc(902)] Could not reach Google Accounts servers: errno -120

Followed by TimeoutException: Timed out while waiting 300s for IsJavaScriptExpressionTrue. No stack traces are available except the javascript exception.

Tried reproducing locally by downloading an isolate and running the tests manually, but no luck. Might have something to do with using the test harness?
Quite a severe regression.

Regression range: http://crrev.com/560158..560185