[anchor] Enable the CSSPositionAnchorNone feature.

This also updates the parsing test to accept "none" and ensure its the
default.

As per resolution in:
w3c/csswg-drafts#13067 (comment)

Bug: 460415950
Change-Id: I411cbf774c2587889478e89ab296c482c3839855

Author: bfgeek

css/css-anchor-position/position-anchor-basics.html

@@ -15,23 +15,24 @@
 
 <script>
 // position-anchor: <anchor-element>
-// <anchor-element> = auto | <dashed-ident>
+// <anchor-element> = none | auto | <dashed-ident>
 test_valid_value('position-anchor', 'auto');
 test_valid_value('position-anchor', '--foo');
-test_invalid_value('position-anchor', 'none');
+test_valid_value('position-anchor', 'none');
 test_invalid_value('position-anchor', 'foo-bar');
 test_invalid_value('position-anchor', '--foo --bar')
 test_invalid_value('position-anchor', '--foo, --bar')
 test_invalid_value('position-anchor', '100px');
 test_invalid_value('position-anchor', '100%');
 
 // Computed value: as specified
+test_computed_value('position-anchor', 'none');
 test_computed_value('position-anchor', 'auto');
 test_computed_value('position-anchor', '--foo');
 
-// Initial: auto
+// Initial: none
 // Inherited: no
-assert_not_inherited('position-anchor', 'auto', '--foo');
+assert_not_inherited('position-anchor', 'none', '--foo');
 
 // Animation type: discrete
 test_no_interpolation({