[A11y] Only allow valid nodes as input into serializer

Tests now pass after the landing of CL:5401359.

Bug: none
Change-Id: I33c2c8c65dc1563f4ee0c3212f8c99c32e80862d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5386012


Reviewed-by: David Tseng <dtseng@chromium.org>
Commit-Queue: Aaron Leventhal <aleventhal@chromium.org>
Auto-Submit: Aaron Leventhal <aleventhal@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1280193}