Bug 1778695 - Account for color-mix pref in test_bug877690.

Differential Revision: https://phabricator.services.mozilla.com/D151370

layout/inspector/tests/test_bug877690.html

@@ -32,7 +32,10 @@ function testValues(values, expected) {
 }
 
 function do_test() {
-  var allColors = [ "COLOR", "color-mix", "currentColor", "transparent", "rgb", "rgba", "hsl", "hsla", "hwb" ];
+  var allColors = [ "COLOR", "currentColor", "transparent", "rgb", "rgba", "hsl", "hsla", "hwb" ];
+  if (SpecialPowers.getBoolPref("layout.css.color-mix.enabled")) {
+    allColors.push("color-mix");
+  }
   var allGradients = [
     "linear-gradient",
     "-webkit-linear-gradient",