Relax the size check in `largest-contentful-paint/image-upscaling.html`

[smfr]

Test List

https://wpt.fyi/results/largest-contentful-paint/image-upscaling.html

Rationale

The test has:

    promise_test(async t => {
      const { naturalSize, lcpSize } = await load_image_and_get_lcp_size(t,
        { transform: 'scale(0.5)' });
      assert_equals(Math.floor(lcpSize), Math.floor(naturalSize / 4));
    }, 'A downscaled image (using scale) should report the displayed size');

Due to rounding errors, WebKit gets 3525 instead of 3524. The test should be revised to allow some slop.

[nt1m]

I merged web-platform-tests/wpt#55083 based on Yoav's review, but happy to reconsider if there's any strong objections to the PR.