The root cause was a bit different from #c0. Preload URLs were cached by other tests in the same directory, and verifyNumberOfDownloads() doesn't count if the resource has came from disk cache.

https://chromium-review.googlesource.com/c/chromium/src/+/1411963/

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/8e4e5d7297698d2a4bc5353a28b511663d28fc40

commit 8e4e5d7297698d2a4bc5353a28b511663d28fc40
Author: Kunihiko Sakamoto <ksakamoto@chromium.org>
Date: Wed Jan 16 08:42:34 2019

Fix preload tests failing on wpt.fyi dashboard

verifyNumberOfDownloads only counts resource timing entries with
transferSize > 0, so it doesn't count if the resource came from a cache.
(See https://bugzilla.mozilla.org/show_bug.cgi?id=1222633#c38 for
background why we have it.)

link-header-preload-nonce.html and link-header-preload-srcset.tentative.html
were failing on wpt.fyi dashboard because they have used the same
subresource URLs as other tests, and preloaded resources from the disk
cache.

This patch adds unique query parameters to these URLs so that they don't
conflict with other tests.

Bug: 922343
Change-Id: I7fd88530f5fdd83ed2a22d2e2893cc446debd036
Reviewed-on: https://chromium-review.googlesource.com/c/1411963
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Kunihiko Sakamoto <ksakamoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#623157}
[modify] https://crrev.com/8e4e5d7297698d2a4bc5353a28b511663d28fc40/third_party/blink/web_tests/external/wpt/preload/link-header-preload-nonce.html
[modify] https://crrev.com/8e4e5d7297698d2a4bc5353a28b511663d28fc40/third_party/blink/web_tests/external/wpt/preload/link-header-preload-nonce.html.headers
[modify] https://crrev.com/8e4e5d7297698d2a4bc5353a28b511663d28fc40/third_party/blink/web_tests/external/wpt/preload/link-header-preload-srcset.tentative.html
[modify] https://crrev.com/8e4e5d7297698d2a4bc5353a28b511663d28fc40/third_party/blink/web_tests/external/wpt/preload/link-header-preload-srcset.tentative.html.headers

Sweet, thanks for fixing this!