Add a WPT for mixed length-percentage CSSTranslate coordinates

Add a WPT for mixed length-percentage CSSTranslate coordinates

Add a regression test covering construction of a CSSTranslate from CSSMathSum
values containing both a length and a percentage. This verifies that mixed
<length-percentage> CSSMathValues are accepted for the X and Y coordinates, as
required by the Typed OM specification.

Differential Revision: https://phabricator.services.mozilla.com/D315277

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=2057579
gecko-commit: d90e27aeba1d29957a5687e42e2483e0ecd868d4
gecko-reviewers: emilio, firefox-style-system-reviewers

Author: janvarga

css/css-typed-om/stylevalue-subclasses/cssTranslate.tentative.html

@@ -65,6 +65,15 @@
   assert_true(result.is2D);
 }, 'CSSTranslate can be constructed from CSSMathValues');
 
+test(() => {
+  const value = new CSSMathSum(CSS.px(-3.14), CSS.percent(3.14));
+  const result = new CSSTranslate(value, value);
+  assert_style_value_equals(result.x, value);
+  assert_style_value_equals(result.y, value);
+  assert_style_value_equals(result.z, CSS.px(0));
+  assert_true(result.is2D);
+}, 'CSSTranslate can be constructed from mixed length-percentage CSSMathValues');
+
 for (const attr of ['x', 'y']) {
   test(() => {
     let result = new CSSTranslate(CSS.px(0), CSS.px(0), CSS.px(0));