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The test is flaky because speculative preloads start, and in some cases
finish loading before disabling console message. Let's use testharness.js
in order to ignore console messages.

Bug: 882292
Change-Id: Ic6627f0bb941e7299ac611c2fd10e6469e1ed5e9
Reviewed-on: https://chromium-review.googlesource.com/1214978Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589832}