We resolve the percentage based on the resolutino of the spec issue [1].
Per the spec issue, if we specify `box-decoration-break:slice`, we have to
use the total length of all breaks as the basis. If the property value
is `clone`, each fragment is considered individually.
Note: text-decoration-inset-030.html is failed because we break the
textframe (when using `<br>`) to avoid some performance issues, so
our rendering result looks like using `box-decoration-break:clone`,
which is not correct. We may have to add some extra handling for the
continouation in the follow-up bugs.
[1] https://github.com/w3c/csswg-drafts/issues/8403
Differential Revision: https://phabricator.services.mozilla.com/D308886
Boris Chiou
f50d8fe9e4