Evict BFCached pages with SharedWorker when the last client navigates away

This CL prevents a SharedWorker from remaining active when all of its
clients are in BFCache. When the last active client of a SharedWorker
navigates away:

- The navigating client is blocked from entering BFCache.
- Any other clients of the same SharedWorker that are already in BFCache are evicted.
- As a result, the SharedWorker is left with no clients and is
subsequently terminated.

This change is the minimum behavior required for the SharedWorkerBFCache
launch as described at "Worker state when last client enters BFCache" in
[1]. The better long-term solution is to freeze the worker instead of
evicting clients, which is noted as TODOs.

[1]
https://docs.google.com/document/d/1dZxwmUYOlDmZ-PF97mfoXbzMrFHBsqeUT3XxTkpNIb0/edit?usp=sharing

Bug: 406420935, 431875857
Change-Id: I038b17dc890e50b7a57e6d8c5f496e11cb302f66
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6914636
Reviewed-by: Anders Hartvoll Ruud <andruud@chromium.org>
Reviewed-by: Guillem Perez <guiperez@google.com>
Auto-Submit: Anna Sato <annasato@chromium.org>
Commit-Queue: Anna Sato <annasato@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1529241}