Bug 1523562 [wpt PR 15075] - Fix preload tests failing on wpt.fyi dashboard, second...

Bug 1523562 [wpt PR 15075] - Fix preload tests failing on wpt.fyi dashboard, second attempt, a=testonly

Automatic update from web-platform-tests
Fix preload tests failing on wpt.fyi dashboard, second attempt

https://crrev.com/c/1411963 fixed the url conflicts with other tests,
but the test still flakes when running repeatedly, because the resource
is loaded from disk cache in subsequent runs
(https://github.com/web-platform-tests/wpt/pull/14882).

We don't care if the preloaded resources came from cache in these tests,
so let's just verify the number of Resource Timing entries.

Bug: 922343
Change-Id: I2c1d146856a125dbbb641d7efb4e6151094f6a3e
Reviewed-on: https://chromium-review.googlesource.com/c/1436777
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Kunihiko Sakamoto <ksakamoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#626019}

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wpt-commits: a52ae0c66f4a764f036d6b85e9d2e611acf45383
wpt-pr: 15075