[IntersectionObserver] Fix testharness API usage

Before this change, the tests did nothing useful :)

Change-Id: Id3e5c18f6d97e9f6a7de23fdf8de3cc0fad3f50d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1580159
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: Stefan Zager <szager@chromium.org>
Cr-Commit-Position: refs/heads/master@{#653731}
2 files changed