[BlinkGenPropertyTrees] Add a note about backdrop filter failures

Backdrop filters are not supported today. This patch makes this clear in
the BlinkGenPropertyTrees expectations file so folks do not spend time
fixing the tests.

Bug: 854189
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I206c40841c9a1992d122206073afa10dd0953625
Reviewed-on: https://chromium-review.googlesource.com/1148896Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577677}

third_party/WebKit/LayoutTests/FlagExpectations/enable-blink-gen-property-trees

@@ -133,6 +133,12 @@ crbug.com/865039 compositing/masks/mask-with-added-filters.html [ Pass ]
 # Link highlights are no longer affected by ancestor effects.
 crbug.com/857501 compositing/gestures/gesture-tapHighlight-with-filter.html [ Pass ]
 
+# Backdrop filters are experimental (behind a flag) and do not really work with
+# or without BlinkGenPropertyTrees. These tests are marked as failing because
+# the expectations differ, but these do not block BlinkGenPropertyTrees.
+crbug.com/854189 css3/filters/backdrop-filter-rendering-no-background.html [ Failure ]
+crbug.com/854189 css3/filters/backdrop-filter-rendering.html [ Failure ]
+
 # These animation/transition tests should pass.
 Bug(none) animations/ [ Pass ]
 Bug(none) transitions/ [ Pass ]
@@ -302,10 +308,6 @@ Bug(none) compositing/overflow/tiled-mask.html [ Failure ]
 # Subpixel differences on text. Likely benign.
 Bug(none) css3/blending/background-blend-mode-overlapping-accelerated-elements.html [ Failure ]
 
-# Backdrop filters are not working.
-crbug.com/854189 css3/filters/backdrop-filter-rendering-no-background.html [ Failure ]
-crbug.com/854189 css3/filters/backdrop-filter-rendering.html [ Failure ]
-
 # Incorrect clipping.
 crbug.com/844429 compositing/culling/tile-occlusion-boundaries.html [ Failure ]