Don't depend on unregisterProperty in register-property-syntax-parsing.html

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

Index: third_party/WebKit/LayoutTests/custom-properties/register-property-syntax-parsing.html
diff --git a/third_party/WebKit/LayoutTests/custom-properties/register-property-syntax-parsing.html b/third_party/WebKit/LayoutTests/custom-properties/register-property-syntax-parsing.html
index b1152311bc4a49a4764229757c25575b4204f056..0195db8e0879e385fea76b27f6089d3f1b04225e 100644
--- a/third_party/WebKit/LayoutTests/custom-properties/register-property-syntax-parsing.html
+++ b/third_party/WebKit/LayoutTests/custom-properties/register-property-syntax-parsing.html
@@ -2,19 +2,21 @@
 <script src="../resources/testharness.js"></script>
 <script src="../resources/testharnessreport.js"></script>
 <script>
+test_count = 0;
+
 function assert_valid(syntax, initialValue) {
     // No actual assertions, this just shouldn't throw
     test(function() {
-        try { CSS.unregisterProperty('--syntax-test'); } catch(e) { }
-        CSS.registerProperty({name: '--syntax-test', syntax: syntax, initialValue: initialValue});
+        var name = '--syntax-test-' + (test_count++);
+        CSS.registerProperty({name: name, syntax: syntax, initialValue: initialValue});
     }, "syntax:'" + syntax + "', initialValue:'" + initialValue + "' is valid");
 }
 
 function assert_invalid(syntax, initialValue) {
     test(function(){
-        try { CSS.unregisterProperty('--syntax-test'); } catch(e) { }
+        var name = '--syntax-test-' + (test_count++);
         assert_throws(new SyntaxError(),
-            () => CSS.registerProperty({name: '--syntax-test', syntax: syntax, initialValue: initialValue}));
+            () => CSS.registerProperty({name: name, syntax: syntax, initialValue: initialValue}));
     }, "syntax:'" + syntax + "', initialValue:'" + initialValue + "' is invalid");
 }