The spec says the behavior is currently undefined and implementations
should not apply the property to generated content. It is still part of
the computed styles, so add a test for that as well.

Bug: 397265
Change-Id: I5391c54c2e3035c90cb5b8db38aa55a77c5ef0b0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2584970
Reviewed-by: Anders Hartvoll Ruud <[email protected]>
Commit-Queue: Rune Lillesveen <[email protected]>
Cr-Commit-Position: refs/heads/master@{#836071}