Align CSSOMVariableReferenceValue serialization with Blink and to match what WPT tests expect.
Pull request: https://github.com/WebKit/WebKit/pull/5915
Committed 256230@main (91bc6304bed2): <https://commits.webkit.org/256230@main> Reviewed commits have been landed. Closing PR #5915 and removing active labels.
<rdar://problem/101861110>