Disable external/wpt/background-fetch/get.https.window.html

This layout test has been flaky for some time.

TBR=peter@chromium.org

Bug: 869470
Change-Id: I1749a81e5172962126c0ee32afaeb7f78e4fca13
Reviewed-on: https://chromium-review.googlesource.com/1157118Reviewed-by: Fabrice de Gans-Riberi <fdegans@chromium.org>
Commit-Queue: Fabrice de Gans-Riberi <fdegans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579519}

third_party/WebKit/LayoutTests/TestExpectations

@@ -4863,3 +4863,6 @@ crbug.com/824539 [ Android ] virtual/layout_ng/fast/block/margin-collapse/019.ht
 # Sheriff 2018-07-30
 crbug.com/866166 virtual/gpu-rasterization/images/color-profile-image-filter-all.html [ Pass Timeout ]
 crbug.com/868706 external/wpt/css/css-layout-api/auto-block-size-inflow.https.html [ Pass Failure ]
+
+# Sheriff 2018-07-31
+crbug.com/869470 external/wpt/background-fetch/get.https.window.html [ Crash Failure ]
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