blink:memory_cache: Fix potential crash from nullptrs in tiered pruning

The new tiered cache pruning logic in PruneTieredStrongReferences was
not robust against nullptr entries, leading to potential crashes.

A nullptr can appear in the tiered_strong_references_ vector because
RemoveInternal was not correctly handling removal for the new code path.
When a resource was removed from the cache, its entry was not being
cleaned up from the tiered vector.

This CL fixes these issues by:
- Updating RemoveInternal to "tombstone" entries in
thetiered_strong_references_ vector by setting them to nullptr.

- Making PruneTieredStrongReferences fully robust by adding safety
checks to handle these nullptr "tombstones" during size calculation,
cleanup, sorting, and eviction.

Bug: 432275223

Change-Id: I8411fdb6aa1463c0c4f3d202a4ff2c824e277ccb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6817961
Commit-Queue: Alex Attar <aattar@google.com>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Reviewed-by: Francois Pierre Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1504577}