Mark external/wpt/css/cssom/idlharness.html as slow test

This patch markes external/wpt/css/cssom/idlharness.html as slow test.

Tbr: yosin@chromium.org
Bug: 1028482
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Change-Id: I4a16f628b3b3e9d3f70dc97300f822ba624c4aa4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1935579
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Commit-Queue: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#719034}

css/cssom/idlharness.html

@@ -4,6 +4,7 @@
 <title>CSSOM automated IDL tests</title>
 <link rel="author" title="Ms2ger" href="mailto:Ms2ger@gmail.com">
 <link rel="help" href="https://drafts.csswg.org/cssom-1/#idl-index">
+<meta name="timeout" content="long">
 <script src=/resources/testharness.js></script>
 <script src=/resources/testharnessreport.js></script>
 <script src=/resources/WebIDLParser.js></script>