Add debugging information when prerendering discards a speculative RFH

This is not expected and will generally lead to a renderer crash, due to
desynced states between the browser process and the render process.

This does *not* fix the existing crashes; it just adds instrumentation
to try to improve the debuggability of this crash in the future.

Bug: 40076091
Change-Id: Ib028f4ba11c15bb1925e7cd387a6d4b680023567
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5891975
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1361890}