[devtools] Look up registered properties for pseudo element nodes

Since devtools represents some pseudo elements like ::before as nodes in the DOM tree, we also need to report and validate registered properties
in the element's own style and not just the inherited styles.

Fixed: chromium:1515814
Change-Id: I49e2132f80e568480d9a9df6c16df38fc07a82b1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5240837
Auto-Submit: Philip Pfaffe <pfaffe@chromium.org>
Reviewed-by: Alex Rudenko <alexrudenko@chromium.org>
Commit-Queue: Alex Rudenko <alexrudenko@chromium.org>
Commit-Queue: Philip Pfaffe <pfaffe@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1253192}