[css-properties-values-api] Fix handling of 'em' unit cycles.

We would incorrectly trigger fallback behavior inside the cycle, which
is not correct. Additionally, the custom property responsible for the
cycle would get 'unset' behavior, which is not correct for custom
properties in a cycle. They should become "invalid at computed-value
time", not unset.

Change-Id: I3865e1c17da06fe7688d2760bde120abff990d85
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1708073
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Commit-Queue: Anders Hartvoll Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/master@{#679505}