[BGPT] Add a flag specific expectation of passing.

This flag specifically improves the results of layout test
compositing/masks/mask-with-added-filters.html


Bug: 865039
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I8589dacff3c77b42931e58bc3d39c0360f93aa9e
Reviewed-on: https://chromium-review.googlesource.com/1142237
Commit-Queue: Peter Mayo <petermayo@chromium.org>
Reviewed-by: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576253}

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

@@ -138,8 +138,7 @@ Bug(none) virtual/threaded/ [ Skip ]
 # These tests failed in normal mode but are fixed with BlinkGenPropertyTrees.
 Bug(none) compositing/overflow/clip-escaping-reverse-order-should-not-crash.html [ Pass ]
 Bug(none) compositing/overflow/composited-scroll-with-fractional-translation.html [ Pass ]
-# This test can be manually rebaselined.
-Bug(none) compositing/masks/mask-with-added-filters.html [ Failure ]
+crbug.com/865039 compositing/masks/mask-with-added-filters.html [ Pass ]
 
 # These animation/transition tests should pass.
 Bug(none) animations/ [ Pass ]