[A11yPerformance][a11y] Move child tree ID storage to AXObjectCache

This CL moves the storage of child tree IDs from
AXObject to AXObjectCacheImpl. This is done
because the child tree ID is not needed by most
AXObjects, and storing it in each object takes up
valuable space.

This improves AXObject memory footprint.

Change-Id: Ic08f54f468c7cd53e436eef80cb3c7611f98d3ad
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6397764
Reviewed-by: Aaron Leventhal <aleventhal@chromium.org>
Commit-Queue: Aaron Leventhal <aleventhal@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1438432}