Mark a flaky test that is blocking wpt import

TBR=robertma@chromium.org

Bug: 1099689
Change-Id: Iba3b63a779a1e2d50716e0a37863177c4546d3fc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2270342Reviewed-by: Luke Z <lpz@chromium.org>
Reviewed-by: Robert Ma <robertma@chromium.org>
Commit-Queue: Luke Z <lpz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#782960}

third_party/blink/web_tests/TestExpectations

@@ -6865,6 +6865,7 @@ crbug.com/867532 [ Linux ] http/tests/workers/worker-usecounter.html [ Pass Time
 # Tests blocking WPT importer
 crbug.com/1098844 external/wpt/wasm/jsapi/functions/entry.html [ Pass Crash Timeout ]
 crbug.com/1098844 external/wpt/wasm/jsapi/functions/entry-different-function-realm.html [ Pass Crash Timeout ]
+crbug.com/1099689 [ Linux ] external/wpt/html/browsers/windows/nested-browsing-contexts/frameElement-siblings.sub.html [ Pass Failure ]
 
 #Sheriff 2020-06-25
 crbug.com/1010170 media/video-played-reset.html [ Pass Failure ]