[A11y] Retain anchor in AXPlatformNodeBase::FindTextBoundary

AsUnignoredPosition has a "last resort" of finding the next/previous
unignored position, but in cases where we are required to stop at Anchor
Boundaries, this behavior can cause bugs, as callers expect the node the
position is anchored in to be retained when called with
AXBoundaryBehavior::kStopAtAnchorBoundary

Bug: 6480215
Change-Id: I89bf02f9f06677750b325cb677ef572e6c3b1b51
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6500379
Commit-Queue: Jacques Newman <janewman@microsoft.com>
Reviewed-by: Benjamin Beaudry <benjamin.beaudry@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1454484}