While debugging, we discovered a Blink misconfiguration in the
navigator.mimeTypes object. We fixed the issue in
https://crrev.com/c/3303674, but let's also document on the V8 side when
you can hit the DCHECK and where to look next.

R=yangguo@chromium.org

Bug: chromium:1262066
Change-Id: I256331ec4296963deb152485d8c6699b75c42e37
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3302804
Auto-Submit: Tim Van der Lippe <tvanderlippe@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Tim Van der Lippe <tvanderlippe@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78154}