[code-health] User pointer refs in AccessibilityTreeFormatterUia

Non-const argument references are confusing. It is not clear that the
methods are modifying the value passed to them.

Bug: 1346006
Change-Id: I5b91f8b68f2bac9be22fbf4ba47bed6fd280de7e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3900464
Commit-Queue: Fabrice de Gans <fdegans@chromium.org>
Reviewed-by: David Tseng <dtseng@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1048083}