Mark css3/filters/effect-reference-image-hw.html as flaky

Tbr: andruud@chromium.org
No-Try: true
Bug: 873454
Change-Id: I713c5266cd9eb12cf49cfe8b952e709f47070ca9
Reviewed-on: https://chromium-review.googlesource.com/1172181Reviewed-by: Makoto Shimazu <shimazu@chromium.org>
Reviewed-by: Anders Ruud <andruud@chromium.org>
Commit-Queue: Makoto Shimazu <shimazu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582516}

third_party/WebKit/LayoutTests/TestExpectations

@@ -3913,6 +3913,7 @@ crbug.com/722943 virtual/video-surface-layer/media/audio-repaint.html [ Skip ]
 # Sheriff failures 2018-08-13
 crbug.com/873547 [ Mac ] virtual/mouseevent_fractional/fast/events/middleClickAutoscroll-event-fired.html [ Timeout Pass ]
 crbug.com/873547 [ Mac ] virtual/user-activation-v2/fast/events/middleClickAutoscroll-event-fired.html [ Timeout Pass ]
+crbug.com/873454 css3/filters/effect-reference-image-hw.html [ Failure Pass ]
 
 crbug.com/715718 external/wpt/media-source/mediasource-activesourcebuffers.html [ Failure Pass ]
 crbug.com/715718 external/wpt/media-source/mediasource-remove.html [ Failure Pass ]