Bug 1873287 [wpt PR 43869] - Do not intercept for prefetch in subframes., a=testonly

Bug 1873287 [wpt PR 43869] - Do not intercept for prefetch in subframes., a=testonly

Automatic update from web-platform-tests
Do not intercept for prefetch in subframes.

Other parts of the prefetch code assume this is for top-level
navigation. If we do want to support this, we'd have to do
adjust it to properly handle cookie and cache partitioning
in a more sophisticated way.

The corresponding test that uses iframes is updated; there is
already parallel coverage using window.open.

Bug: 1511780
Change-Id: I249432e6a5de4b6ffbf1d7c6c260a78850151d3b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5173989
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Reviewed-by: Hiroshige Hayashizaki <hiroshige@chromium.org>
Commit-Queue: Kouhei Ueno <kouhei@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1245137}

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wpt-commits: ed581aee5c764bea75a51dd6d1f113a1580962ba
wpt-pr: 43869