Remove stale comment about typedcssom/cssKeywordValue.html failing

This patch removes a stale comment in typedcssom/cssKeywordValue.html,
the bug was fixed previously by codereview.chromium.org/1778743003.

BUG=584999

Review-Url: https://codereview.chromium.org/2024773002
Cr-Commit-Position: refs/heads/master@{#397039}

third_party/WebKit/LayoutTests/typedcssom/cssKeywordValue.html

@@ -7,7 +7,6 @@ test(function() {
   assert_throws(TypeError(), function() { new CSSKeywordValue("") });
 }, "Constructor should throw an error if given an empty string");
 
-//This test fails due to bug: 584999
 test(function() {
   assert_equals(new CSSKeywordValue('initial').cssString, 'initial');
   assert_equals(new CSSKeywordValue('center').cssString, 'center');