Mark WPT's infrastructure/assumptions/blank.html as failing/crashing

When DCHECKs are enabled, this test crashes the test runner (but not when
content_shell is run manually) and blocks WPT imports. When DCHECKs are
disabled, the runner treats it as a pixel test and produces wrong results.

TBR=foolip, robertma

Bug: 895777
Change-Id: Ia0cb5bcbfb56a67e10bfd93a01ab6995fb02d328
No-Try: True
Reviewed-on: https://chromium-review.googlesource.com/c/1283130Reviewed-by: Raphael Kubo da Costa (CET) <raphael.kubo.da.costa@intel.com>
Reviewed-by: Robert Ma <robertma@chromium.org>
Commit-Queue: Raphael Kubo da Costa (CET) <raphael.kubo.da.costa@intel.com>
Cr-Commit-Position: refs/heads/master@{#599937}

third_party/WebKit/LayoutTests/TestExpectations

@@ -5323,3 +5323,6 @@ crbug.com/891155 [ Linux ] virtual/user-activation-v2/fast/events/middleClickAut
 crbug.com/895257 [ Mac ] external/wpt/css/css-fonts/variations/at-font-face-font-matching.html [ Failure Pass ]
 crbug.com/891155 [ Linux ] fast/events/middleClickAutoscroll-event-fired.html [ Failure Pass ]
 crbug.com/891155 [ Linux ] fast/events/middleClickAutoscroll-drag.html [ Failure Pass ]
+
+# ecosystem-infra sheriff 2018-10-16
+crbug.com/895777 external/wpt/infrastructure/assumptions/blank.html [ Failure Crash ]