This happens on at least the Mac trybots, probably all of them.

https://build.chromium.org/p/tryserver.chromium.mac/builders/mac_chromium_rel_ng/builds/230587/steps/webkit_tests%20%28with%20patch%29/logs/stdio failed 4 new tests, but it only included "asserts failed" in the output instead of the actual error message:

23:52:39.778 30161 worker/7 bluetooth/gattserverdisconnected-event/disconnected.html output stderr lines:
23:52:39.779 30161   [30916:1287:0518/235239:1946892998130:WARNING:bluetooth_dispatcher_host.cc(554)] No Bluetooth chooser implementation; falling back to first device.
23:52:39.780 30126 [6132/40979] bluetooth/gattserverdisconnected-event/disconnected.html failed unexpectedly (asserts failed)
23:52:39.779 30161 worker/7 bluetooth/gattserverdisconnected-event/disconnected.html failed:
23:52:39.779 30161 worker/7  asserts failed


Running `"out/rel/Content Shell.app/Contents/MacOS/Content Shell" --run-layout-test third_party/WebKit/LayoutTests/bluetooth/gattserverdisconnected-event/disconnected.html` on an actual Mac reveals most of the real error:

ontent-Type: text/plain
This is a testharness.js-based test.
FAIL A device disconnecting while connected should fire the gattserverdisconnected event. promise_test: Unhandled rejection with value: object "NotSupportedError: getCharacteristic is not implemented y..."
Harness: the test ran to completion.


However, it can take hours to finish a build even when you already have a Mac laptop. The trybot should show this text.