Add basic support for enumerating computed styles (#31499)

* Add basic support for enumerating computed styles

The old code was so broken, it was enumerating the style attribute
instead of the computed styles.

So this patch implements a basic enumeration, but not including custom
properties whose computed value is not the guaranteed-invalid value.

* Feedback

tests/wpt/meta/css/css-cascade/all-prop-revert-layer.html.ini

@@ -1,2 +1,12 @@
 [all-prop-revert-layer.html]
-  expected: TIMEOUT
+  [block-size]
+    expected: FAIL
+
+  [inline-size]
+    expected: FAIL
+
+  [width]
+    expected: FAIL
+
+  [height]
+    expected: FAIL