[Flaky] Mark long_idle_periods.html as prone to timing out

This test is known to be flaky and a change related to it didn't change
that (see linked bug).

TBR=keishi@chromium.org

Bug: 862589
Change-Id: I3c64042bb3d36c9515d2b1b302d13b85fbbc8034
Reviewed-on: https://chromium-review.googlesource.com/1180963
Commit-Queue: Friedrich Horschig <fhorschig@chromium.org>
Reviewed-by: Friedrich Horschig <fhorschig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584398}

third_party/WebKit/LayoutTests/TestExpectations

@@ -5005,4 +5005,7 @@ crbug.com/875287 fast/frames/crash-frameset-CSS-content-property.html [ Crash ]
 # Wake Lock api test timeouts
 crbug.com/872530 external/wpt/wake-lock/wakelock-applicability-manual.https.html [ Pass Timeout ]
 crbug.com/872530 external/wpt/wake-lock/wakelock-document-hidden.https.html [ Pass Timeout ]
-crbug.com/872535 external/wpt/wake-lock/wakelock-state-is-global.https.html [ Pass Timeout ]
+crbug.com/872535 external/wpt/wake-lock/wakelock-state-is-global.https.html [ Pass Timeout ]
\ No newline at end of file
+
+# Sheriff 2018-08-20
+crbug.com/862589 [ Linux Mac Win ] virtual/threaded/fast/idle-callback/long_idle_periods.html [ Timeout Pass ]