create a sum value missing CSSMathClamp

[achristensen07]

Seems to be implemented in Chromium and missing in https://drafts.css-houdini.org/css-typed-om/#create-a-sum-value

[tabatkins]

Oh, indeed. Thanks for the catch.

[achristensen07]

Same with https://drafts.css-houdini.org/css-typed-om/#equal-numeric-value