Make attr() taint stick through registered custom properties.

Change-Id: I1ce0317df871090b28230eb8f83eaf8306210c4a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5852603


Reviewed-by: Anders Hartvoll Ruud <andruud@chromium.org>
Reviewed-by: Steinar H Gunderson <sesse@chromium.org>
Commit-Queue: Steinar H Gunderson <sesse@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1358725}