Make CSS Typed OM to_string() implementations produce Utf16String
instead of building UTF-8 strings and converting at the JS binding
boundary. Use Utf16StringBuilder for numeric, math, and transform
serialization paths that already feed CSS Typed OM stringification.
Keep the older CSSOM StyleValue serializer on StringBuilder for now, and
convert only where Typed OM source values or parser re-entry still need
the existing UTF-8 representation.
DOMMatrix.to_string() throws exceptions if any of its values are
non-finite. This ends up affecting CSSStyleValue because its subclass
CSSTransformValue (which is about to be added) serializes
CSSTransformComponents, and one of those is CSSMatrixComponent, which
calls DOMMatrix.to_string().
This is all quite unfortunate, and because at the time the spec for
DOMMatrix was written, CSS couldn't represent NaN or infinity. That's
no longer true, so I'm hoping the spec can be updated and this can be
reverted. https://github.com/w3c/fxtf-drafts/issues/611
We don't serialize these the way WPT expects, because we don't implement
the comment-insertion rules from CSS-Syntax. We don't implement that
for regular serialization either, so it's something we can worry about
later.
