Robust handling of empty embed objects

This is a required precursor for synchronous a11y serialization in CL:3914885. Sometimes an empty embed or object is serialized earlier that it would have been. Make sure that they are always marked ignored/disabled/non-focusable, and that dump tree tests don't wait for them to have children.

Bug: none
NOTRY: true
Change-Id: I8e3309d428ec991108e22edb650c81e9ba78680f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4021995
Commit-Queue: Aaron Leventhal <aleventhal@chromium.org>
Reviewed-by: David Tseng <dtseng@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1071693}