Auto merge of #17284 - jyc:custom-properties-cascade, r=emilio

Propagate changes in custom properties' computed values to descendants

If ComputedValues.custom_properties differs between the old and new
ComputedValues, indicate that we have to propogate changes to
descendants by setting child_cascade_requirement to
MustCascadeDescendants in cascade_primary.

style::matching::TElement::cascade_primary already calls
accumulate_damage, which eventually calls
ServoRestyleDamage::compute_style_difference in order to check if other
properties' computed values changed. If any of those change, we signal
that we need to propogate changes for inherited properties.

With Properties & Values, some custom properties will not be inherited,
and we will need to revisit this.

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tests/wpt/mozilla/meta/MANIFEST.json

@@ -20015,6 +20015,12 @@
      {}
     ]
    ],
+   "mozilla/upstream/css_variables_setProperty_recompute.html": [
+    [
+     "/_mozilla/mozilla/upstream/css_variables_setProperty_recompute.html",
+     {}
+    ]
+   ],
    "mozilla/variadic-interface.html": [
     [
      "/_mozilla/mozilla/variadic-interface.html",
@@ -31700,6 +31706,10 @@
    "dfb31d3f9c4f24913055924c375f08b990b63e49",
    "testharness"
   ],
+  "mozilla/upstream/css_variables_setProperty_recompute.html": [
+   "2e9326948f5115b3b63564fd26bae326bc8e2187",
+   "testharness"
+  ],
   "mozilla/variadic-interface.html": [
    "9edd5150d36fabae42077a034655a8457eb75bff",
    "testharness"