Fenced frames: Verify script speculationrules prefetch is not
supported

Script speculationrules prefetch is not supported in fenced frames
because `PrefetchDocumentManager::CanPrefetchNow` always block such
requests from fenced frames.

An example of speculationrules prefetch:
<script type="speculationrules">
  {
    "prefetch": [{"urls": ["next.html"]}]
  }
</script>

Bug: 1515599
Change-Id: Iacb121516ed36ccf5749eceddf8e4b777e59b318
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5769830
Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Commit-Queue: Xiaochen Zhou <xiaochenzh@chromium.org>
Reviewed-by: Adithya Srinivasan <adithyas@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1341921}