Style system and plumbing for mix-blend-mode: plus-lighter.

Differential Revision: https://phabricator.services.mozilla.com/D137951

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1746248
gecko-commit: e4d4cbdd2bea6aaaca590320e5dc3d7414758f23
gecko-reviewers: jrmuizel, layout-reviewers, boris

css/compositing/mix-blend-mode/mix-blend-mode-parsing.html

@@ -22,7 +22,7 @@
             }
 
             var blendModes = ["normal", "multiply", "screen", "overlay", "darken", "lighten", "color-dodge", "color-burn",
-                      "hard-light", "soft-light", "difference", "exclusion", "hue", "saturation", "color", "luminosity"];
+                      "hard-light", "soft-light", "difference", "exclusion", "hue", "saturation", "color", "luminosity", "plus-lighter"];
 
             for (var i = 0; i < blendModes.length; i++) {
                 test(function() {assert_equals(testParse("mix-blend-mode: " + blendModes[i]), blendModes[i] )}, "Mix-blend-mode " + blendModes[i]);