[A11y] Add Size() to AXBitSet

AX enum attributes (StringAttribute, IntAttribute, etc) within
AXNodeData are stored in vectors, which include a Size() function. To
maintain consistency when boolean attributes are stored using AXBitset,
add a Size() function to AXBitset.

Change-Id: I9aa971945ed9f42901fc371e150b9879034f1a4a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6557426
Reviewed-by: Kevin Ellis <kevers@chromium.org>
Commit-Queue: Aaron Leventhal <aleventhal@chromium.org>
Reviewed-by: Aaron Leventhal <aleventhal@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1464303}