Mark external/wpt/svg/idlharness.window.js as slow.

Bug: 888225
Change-Id: Ife7943d9e409c9cb6441ad8b83046de14ad1a3ca
No-Try: true
Tbr: jbroman@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/1241093Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Commit-Queue: Jeremy Roman <jbroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593630}

third_party/WebKit/LayoutTests/SlowTests

@@ -1446,3 +1446,4 @@ crbug.com/878650 [ Release ] inspector-protocol/heap-profiler/heap-snapshot-merg
 crbug.com/878650 [ Release ] inspector-protocol/heap-profiler/heap-snapshot-with-no-detached-iframe.js [ Slow ]
 
 crbug.com/878995 [ Win ] external/wpt/async-local-storage/key-types.tentative.https.html [ Slow ]
+crbug.com/888225 external/wpt/svg/idlharness.window.html [ Slow ]