Keep an extra BFCache entry position for prioritized pages

When a BFCache entry of prioritized page (currently search result page
of the default search engine) needs to be evicted, it will be kept in
the cache instead. Only the latest prioritized entry outside the limit
will be handled in this way.

More background and information can be found from:
https://docs.google.com/document/d/1SHhL4TaiSlXU3xzG_Oi_G3uOyPANSwEz3I6IQME1Tk8/edit

Bug: 402313221
Change-Id: Ibd65a6c3a511826a091b58296c8106a79b92cf09
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6347722
Reviewed-by: Fergal Daly <fergal@chromium.org>
Commit-Queue: Mingyu Lei <leimy@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1434125}