Mark external/wpt/longtask-timing/longtask-in-sibling-iframe-crossorigin.html flaky

TBR=kinuko@chromium.org

Bug: 1025123
Change-Id: I467a196df9286b088516b43b6deb8e7d3acf433a
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1918790Reviewed-by: Hajime Hoshi <hajimehoshi@chromium.org>
Commit-Queue: Hajime Hoshi <hajimehoshi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715589}

third_party/blink/web_tests/TestExpectations

@@ -5847,3 +5847,6 @@ crbug.com/963183 http/tests/devtools/sources/debugger-pause/set-return-value.js
 crbug.com/963183 http/tests/devtools/sources/debugger/debugger-proto-property.js [ Pass Failure ]
 
 crbug.com/892983 external/wpt/css/css-pseudo/first-line-with-before-after.html [ Failure ]
+
+# Sheriff 2019-11-15
+crbug.com/1025123 external/wpt/longtask-timing/longtask-in-sibling-iframe-crossorigin.html [ Pass Failure ]