This seems to have started since
https://chromium-review.googlesource.com/885684

Meanwhile I'm going to mark the test to be prone to time out.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/e668fcee7edb0b9c70bb708a9977e4d9bad47432

commit e668fcee7edb0b9c70bb708a9977e4d9bad47432
Author: Samuel Huang <huangs@chromium.org>
Date: Mon Jan 29 21:53:07 2018

[Sheriff] Mark webkit_layout_tests v/o-c/e/w/s-w/s-w/fetch-event.https.html as prone to Timeout.

TBR: falken@chromium.org
No-Try: true
Bug:  807014 
Change-Id: Iac0c9a8e7b4959c0848cc3722f96d15f306877dc
Reviewed-on: https://chromium-review.googlesource.com/891484
Commit-Queue: Samuel Huang <huangs@chromium.org>
Reviewed-by: Samuel Huang <huangs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532605}
[modify] https://crrev.com/e668fcee7edb0b9c70bb708a9977e4d9bad47432/third_party/WebKit/LayoutTests/TestExpectations

 Issue 806953  has been merged into this issue.

 Issue 806992  has been merged into this issue.

From dups: external/wpt/service-workers/service-worker/fetch-event.https.html and virtual/service-worker-script-streaming/external/wpt/service-workers/service-worker/fetch-event.https.html are also timeout-flaky.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/5fa556bd0c523735707a695abc50d7596c0bf92c

commit 5fa556bd0c523735707a695abc50d7596c0bf92c
Author: Bret Sepulveda <bsep@chromium.org>
Date: Tue Jan 30 00:00:00 2018

Mark all versions of fetch-event.https.html as timeout-flaky.

TBR=falken@chromium.org

Bug:  807014 
Change-Id: I14f0ed5edfe3ace787c74b9f518c46c154477a45
Reviewed-on: https://chromium-review.googlesource.com/892038
Reviewed-by: Bret Sepulveda <bsep@chromium.org>
Commit-Queue: Bret Sepulveda <bsep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532673}
[modify] https://crrev.com/5fa556bd0c523735707a695abc50d7596c0bf92c/third_party/WebKit/LayoutTests/TestExpectations

Link to the flakiness dashboard:
https://test-results.appspot.com/dashboards/flakiness_dashboard.html#testType=webkit_layout_tests&tests=%2Ffetch-event.https.html

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/b72e89031fb32003d290940c1d95a6ed0af36302

commit b72e89031fb32003d290940c1d95a6ed0af36302
Author: Matt Falkenhagen <falken@chromium.org>
Date: Tue Jan 30 04:51:09 2018

service worker: Deflake fetch-event.https.html.

The test was using add_result_callback() which is triggered after a test
finishes. Unfortunately this file uses both async_test and promise_test.
promise_tests run sequentially but async_tests run in parallel. Therefore,
the add_result_callback was being called every time an async_test finished,
causing a race when this promise_test was in the middle of execution.

R=shimazu

Bug:  807014 
Change-Id: Ic1bc568fe812f540feec901f95cc89684ef1cae2
Reviewed-on: https://chromium-review.googlesource.com/892553
Reviewed-by: Makoto Shimazu <shimazu@chromium.org>
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532758}
[modify] https://crrev.com/b72e89031fb32003d290940c1d95a6ed0af36302/third_party/WebKit/LayoutTests/TestExpectations
[modify] https://crrev.com/b72e89031fb32003d290940c1d95a6ed0af36302/third_party/WebKit/LayoutTests/external/wpt/service-workers/service-worker/fetch-event.https.html

 Issue 807219  has been merged into this issue.