Mark external/wpt/compat/idlharness.window.html flaky

This sometimes times out on a Linux bot.

TBR=kinuko@chromium.org

No-Try: true
Bug: 1017626
Change-Id: I53be2feafa8de6a2f6d1768f59e7c9eea4e7e457
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1875901Reviewed-by: Hajime Hoshi <hajimehoshi@chromium.org>
Commit-Queue: Hajime Hoshi <hajimehoshi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#708943}

third_party/blink/web_tests/TestExpectations

@@ -5727,3 +5727,6 @@ crbug.com/1016762 external/wpt/html/cross-origin-opener-policy/popup-redirect-ca
 
 # cci-trooper 2019-10-22
 crbug.com/1016804 external/wpt/css/css-grid/layout-algorithm/grid-flex-track-intrinsic-sizes-002.html [ Pass Failure ]
+
+# Sheriff 2019-10-24
+crbug.com/1017626 [ Linux ] external/wpt/compat/idlharness.window.html [ Pass Timeout ]