Mark external/wpt/selection/idlharness.window.html slow

This patch marks "external/wpt/selection/idlharness.window.html" slow.

TBR=yosin@chromium.org

Bug: 1048204
No-Tree-Checks: true
No-Try: true
Change-Id: Ibce06c7bb63790ff7c97d060e64c99e16e14755f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2038294Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Commit-Queue: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#738431}

third_party/blink/web_tests/SlowTests

@@ -672,6 +672,8 @@ crbug.com/1020379 http/tests/devtools/a11y-axe-core/* [ Slow ]
 crbug.com/1040054 http/tests/devtools/cache-storage/cache-data.js [ Slow ]
 crbug.com/1040450 http/tests/devtools/persistence/persistence-external-change-breakpoints.js [ Slow ]
 
+crbug.com/1048204 external/wpt/selection/idlharness.window.html [ Slow ]
+
 # Reaching the stack size limit makes for a consistently slow test with
 # occasional timeouts on Win7
 crbug.com/937546 [ Win7 ] http/tests/security/xss-DENIED-cross-origin-stack-overflow.html [ Slow ]