Remove CSS.unregisterProperty as the WG resolved to remove it for now

third_party/WebKit/LayoutTests/custom-properties/register-property.html

Index: third_party/WebKit/LayoutTests/custom-properties/register-property.html
diff --git a/third_party/WebKit/LayoutTests/custom-properties/register-property.html b/third_party/WebKit/LayoutTests/custom-properties/register-property.html
index 7a5aadadcdd834a21a1c346d951ba2f0f8e9885c..5c8b45b9e91f3817805c07382bc5673b880f83cc 100644
--- a/third_party/WebKit/LayoutTests/custom-properties/register-property.html
+++ b/third_party/WebKit/LayoutTests/custom-properties/register-property.html
@@ -32,4 +32,10 @@ test(function() {
     assert_throws(new SyntaxError(),
         () => CSS.registerProperty({name: '--syntax-test-3', syntax: 'length'}));
 }, "registerProperty only allows omitting initialValue is syntax is '*'");
+
+test(function() {
+    CSS.registerProperty({name: '--re-register', syntax: '<length>', initialValue: '0px'});
+    assert_throws({name: 'InvalidModificationError'},
+        () => CSS.registerProperty({name: '--re-register', syntax: '<percentage>', initialValue: '0%'}));
+}, "registerProperty fails for an already registered property");
 </script>