Seems related to https://chromium-review.googlesource.com/c/chromium/src/+/625035. Assigning to qyearsley.

You're right, thanks for reporting.

Looks like I made a mistake in that CL, and after that there is no space before the error message in testharness results.

OK, I am going to disable the test in the meantime.

I see you already have CL in progress.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/6deecc296c74178abfb08df78fcb4293db8facd6

commit 6deecc296c74178abfb08df78fcb4293db8facd6
Author: Quinten Yearsley <qyearsley@google.com>
Date: Mon Aug 28 23:09:51 2017

Temporarily suppress failure for header-content-type.html

Bug:  759825 
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Change-Id: I42999cdf1f825aa77625d906e1ade80b06d819b2
Reviewed-on: https://chromium-review.googlesource.com/639633
Commit-Queue: Tarun Bansal <tbansal@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#497923}
[modify] https://crrev.com/6deecc296c74178abfb08df78fcb4293db8facd6/third_party/WebKit/LayoutTests/TestExpectations

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/b7937aba09157a1b21154f4458eeb736126c9cbb

commit b7937aba09157a1b21154f4458eeb736126c9cbb
Author: Quinten Yearsley <qyearsley@chromium.org>
Date: Wed Sep 13 19:16:06 2017

Remove flaky expectation for header-content-type.html

This reverts things back to the way they were before
I incorrectly associated this test's timeout with
a different bug.

Flakiness dashboard link:
https://test-results.appspot.com/dashboards/flakiness_dashboard.html#testType=webkit_tests&tests=external/wpt/beacon/headers/header-content-type.html

Bug: 626703,  759825 
Change-Id: I331be7240bcf806e48504e4a13cf09f0909cfb07
Reviewed-on: https://chromium-review.googlesource.com/660838
Commit-Queue: Quinten Yearsley <qyearsley@chromium.org>
Reviewed-by: Takeshi Yoshino <tyoshino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#501710}
[modify] https://crrev.com/b7937aba09157a1b21154f4458eeb736126c9cbb/third_party/WebKit/LayoutTests/TestExpectations

Update: This test is just slow and sometimes times out. If the only problem is that it's a bit slow, then it could be split into multiple tests.

Currently in TestExpectations it's associated with the general cover bug for all untriaged tests (bug 626703) so I'll close this bug.

tyoshino@, feel free to reopen this bug if you'd like to change/fix this test :-)

http://w3c-test.org/beacon/headers/header-content-type.html