[css-typed-om] ay which is 0 should omit when serialize a CSSSkew

when ay value is 0, second value should omit when serialized.

https://drafts.css-houdini.org/css-typed-om-1/#serialize-a-cssskew

Bug: 808321
Change-Id: I933a42d46b12578acf66ca2065c4761319cb086b
Reviewed-on: https://chromium-review.googlesource.com/899703
Commit-Queue: Hwanseung Lee <hs1217.lee@samsung.com>
Reviewed-by: Darren Shen <shend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#534313}

css/css-typed-om/stylevalue-serialization/cssTransformValue.tentative.html

@@ -44,6 +44,11 @@
     cssText: 'skew(90deg, 45deg)',
     desc: 'CSSSkew'
   },
+  {
+    value: new CSSSkew(CSS.deg(90), CSS.turn(0)),
+    cssText: 'skew(90deg)',
+    desc: 'CSSSkew with Y which is 0 value'
+  },
   {
     value: new CSSPerspective(CSS.px(1)),
     cssText: 'perspective(1px)',