[css-typed-om] values attribute should be readonly

values attribute should be immutable to prevent cyclic
references in CSSMathMax, CSSMathMin, CSSMathProduct,
CSSMathSum.

Bug: 803687
Change-Id: Ibb93165ffd7a279a80febba0543ac186b67dbe56
Reviewed-on: https://chromium-review.googlesource.com/890558
Reviewed-by: Darren Shen <shend@chromium.org>
Commit-Queue: Hwanseung Lee <hs1217.lee@samsung.com>
Cr-Commit-Position: refs/heads/master@{#532336}