[css-typed-om] Don't crash for unrepresentable shorthands

ComputedStylePropertyMap::SerializationForShorthand for some reason expects that CSSValueFromComputedStyle can't return nullptr, when in reality this can easily happen as soon as you have a shorthand which can't represent the currently held longhands.

We have likely exercised this code for a long time, but ever since CL:5577948, we began crashing, since NOTREACHED() is fatal now.

Looking briefly over the spec, it is not defined what to return in this situation, hence adding a crash test which doesn't make assertions about the return value.

Fixed: 364443014 Change-Id: I5c6bcc4ef75285194aed2fc0a6b57429655c7610 Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5840236 Reviewed-by: Rune Lillesveen futhark@chromium.org Commit-Queue: Anders Hartvoll Ruud andruud@chromium.org Cr-Commit-Position: refs/heads/main@{#1352461}