PA: Add 4S seam for `PartitionBucket`

This CL is intended to be trivial. It introduces a new, default-false
condition to initializing `PartitionBucket::can_store_raw_size`,
allowing it to use a more nuanced condition for determining "single-slot
spans" (but not actually using it as of this CL).

A later CL will hook it up to Chromium's experimentation framework,
allowing this condition to activate in the experiment arm.

Bug: 333443437
Change-Id: Ic9ee2e9d0f258b478927e4b8b74b1dc977b0ce0f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5550260
Auto-Submit: Kalvin Lee <kdlee@chromium.org>
Reviewed-by: Takashi Sakamoto <tasak@google.com>
Commit-Queue: Kalvin Lee <kdlee@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1303236}