PA: Make GetZeroSegmentSize() public

Bug: 40925855

Change-Id: Ib0a0663eebec29c0abaa97f6c5ea47c45f75bab8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7850604
Commit-Queue: Takashi Sakamoto <tasak@google.com>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1632522}
NOKEYCHECK=True
GitOrigin-RevId: 58c6cb30b313a31447ad889086e8efe29f43ea1b