Prevent access to an unset optional in BFCache Metrics.

This happens when user navigates forwards and backwards quickly, before
`MainFrameDidNavigateAwayFromDocument` is called AND for the renderer is
killed.

It seems like this should be quite rare but we have at least one crash
report. This might bewhat was generating the previous crashes since it's
the crash looks the same: `SIGABRT @0x00000000`.

Also upgrade a DCHECK to a CHECK. There was a DCHECK for the bad
timestamp and if it had been a CHECK, debugging this would have been
much easier.

Bug: 427426299
Change-Id: I04f9436e604f15dbdb779de7bac600ab7010fea8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6731580
Commit-Queue: Fergal Daly <fergal@chromium.org>
Reviewed-by: Rakina Zata Amni <rakina@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1486810}